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Guide to clean­ing XS650 car­bur­etors

This guide is for the XS650, but pretty much applies to any­thing.

Ori­gin­al PDF here: XS 650 car­bguide
This manu­al has been pre­pared by Grizld1 (aka Dick Rus­sell, Yamaha 650 Soci­ety
Tech Advisor) and 5twins. Mater­i­al has been peer reviewed by Gar­age staff (thanks,
y’all!), and is as accur­ate as we know how to make it.
We’ve tried to gath­er the most fre­quently-reques­ted inform­a­tion into a single ref­er­ence.
In the pro­cess, we’ve can­ni­bal­ized our own posts ruth­lessly. We do not offer this guide
as the last word on XS650 car­bur­etion; ref­er­ences online and in print that we’ve found
valu­able have been recom­men­ded, and we hope you find them use­ful.
Please note: Dir­ec­tion­al terms (right/​left, forward/​back, above/​below) refer to pos­i­tion
from the per­spect­ive of the rider in the saddle.
For ease of use, each major sec­tion below appears in a sep­ar­ate post.

I. Tools: Post 2
II. OEM Carbs, BS38 (70−79) and BS34 (80−83): Clean­ing, Adjust­ment, Tun­ing
…Post 3. Car­bur­etor remov­al and install­a­tion
…Post 4. Floats and slides
…Post 5. Pilot cir­cuit
.…..a. pilot jets
.…..b. air and mix­ture pas­sages
.…..c. mix­ture screws
…Post 6. Main cir­cuit, Enrich­er (Choke), Syn­chron­iz­ing
.…..a. main jets
.…..b. needle jets and needles
.…..c. Enrich­er (choke)
.…..d. Syn­chron­iz­ing
…Post 7. Seal­ing Air Leaks
.…..a. Find­ing air leaks
.…..b. Seal­ing mount­ing boots
.…..c. Seal­ing throttle shafts
…Post 8. Tun­ing for Modi­fic­a­tions
…Addendum March 9, 2009 Poten­tial Float Sys­tem Defect
…Post 9. Troubleshoot­ing
…Post 10. Appendix
.…..1. Chart, spe­cific­a­tions
.…..2. Basic CV carb oper­a­tion: schem­at­ic
.…..3. Starter sys­tem: schem­at­ic
.…..4. Pilot jets, com­par­is­on chart
.…..5. Starter pickup tube, photo
.…..6. BS34 Air jets, photo
.…..7. BS38 Air jets, photo
.…..8. Pilot jets and needle, photo
.…..9. BS38 Mix screws & vacu­um ports, photo
.….10. BS34 Mix screw loc­a­tion, photo
.….11. Mix screw & by-pass out­lets, photo
.….12. Idle & sync adjust­ment screws, photo
.…13. BS38 Float Vari­ations
Wan­ing! Rebuild kits are pro­duced cheaply to fit a vari­ety of applic­a­tions, and some
parts found in them will not be cor­rect for your applic­a­tion. Always check parts against
OEM com­pon­ents before installing. Mike’s XS is very clear about this but many vendors
are not, assum­ing that buy­ers already know. The issue isn’t that rebuild kits are
defect­ive; they’re gen­er­ic, and not everything in them is inten­ded to fit every applic­a­tion
lis­ted for the kits.


Post 2: TOOLS


Jets with slot­ted heads are eas­ily deformed, and prob­lems can be avoided by grind­ing
screw­drivers to achieve a tight, no-slop fit in main and pilot jets. Screw­drivers and bits
with tapered blades tend to apply pres­sure only at the upper edges of the slot; hol­low
ground screw­drivers make con­tact equally from top to bot­tom, and are less likely to
dam­age jets. If your carbs have float bowl plugs, main jets can be installed and
removed without remov­ing float bowls. For BS series, use a 14″ drive bit, ground as
above, and turn down the shank until it clears. Turn it with a 14″ box end wrench, or
5twins’ weapon of choice, a Sears fin­ger bit driver (http://​tiny​url​.com/​y​9​x​ar5). 5twins
also notes that Wiha 2″ met­ric power bits in 4 mm (pilots), 6.5 mm (small round mains)
and (pos­sibly, this is untried) 8 mm. (large round mains) are a good fit; see
www​.wiha​t​ools​.com For VM carbs, use a 6 mm. 14″ drive deep sock­et and hex drive
adaptor for the main jets, turn­ing with a 14″ box end wrench. Set the screw­drivers and
bits aside for use only in carb work.

Cal­iper or Steel Rule.

A cal­iper is use­ful for meas­ur­ing float height (see Sec­tion II. or Sec­tion III). The end of
the depth probe can be set on the carb body while the corner of the shank is sighted on
the float top. Some prefer to sight the float top with a small steel rule.

Fuel Level Sight Tube.

A dir­ect read­ing of fuel level can be taken with a sight tube. Find a rub­ber plug to fit the
plug port in the bot­tom of the float bowl. Drill it and fit it with a hard plastic tube or hose
barb,and attach about 8″ of clear plastic hose. Some BS-series carbs are fit­ted with a
drain valve in the float bowl, opened by a screw, with a nipple for a drain tube, to which
a sight tube can be attached. See Sec­tion II. for use.
Mano­met­er (Syn­chron­izer).

XS650’s after 1976 are fit­ted with vacu­um ports in the carb bod­ies, vacu­um line barbs in
the mount­ing boots, or both. Syn­chron­izers can be con­nec­ted to either loc­a­tion (see
Sec­tion II. or III.)
Inex­pens­ive dial gauges should be avoided. Mer­cury mano­met­ers (“mer­cury sticks”) are
com­mer­cially avail­able, but mer­cury is highly tox­ic and the device is eas­ily broken.
Mor­gan makes the Carb­tune syn­chron­izer, employ­ing steel rods, but it is lim­ited in
accur­acy and expens­ive.
A very accur­ate and dur­able tool can be made with a yard­stick, 10 ft. of 316″ clear
plastic tubing, a few ounces of auto­mat­ic trans­mis­sion flu­id (some folks prefer 2‑stroke
oil), and a pair of pilot jets (the long-pat­tern jets for Mikuni flat slide carbs, sizes 15 to
35, work nicely).
Loop the hose around the bot­tom of the yard­stick and secure it with wire or twist ties so
that it runs straight up both sides; avoid pinch­ing. Trim the ends to the same length, and
cut an 8″ piece from each end. Suck ATF into the line so that it fills the tubing about 8″
high on each side. Splice the end pieces back on with the pilot jets.
Remote Fuel Bottle.

On OEM carbs with a cent­ral syn­chron­iz­ing screw,adjustment (see Sec­tion II.) is easi­est
with the tank removed. Make a remote tank from a 1 qt. plastic bottle. Install a 14″ hose
barb in the bot­tom of the bottle and a lawn mower fuel valve in the line (an in-line fil­ter is
also a good idea). Attach a wire to the top of the bottle and hang it where you need it.
Safety note: many plastics will break down with expos­ure to fuel. Be sure your mater­i­al
is fuel res­ist­ant, and empty the con­tain­er imme­di­ately after use.
This item is not strictly neces­sary. 5twins recom­mends rough adjust­ment with the tank
removed, using fuel remain­ing in the float bowls, fol­lowed by fine adjust­ment with the
tank in place, reach­ing in with a long screw­driver. You can also set the tank on a work
bench or oth­er elev­ated sur­face and run exten­sions to the car­bur­etor lines.

If you intend to do much mech­an­ic­al work at all, an air com­pressor with suf­fi­cient
capa­city to power an impact wrench will pay for itself quickly. For carb clean­ing, attach
an air gun with a rub­ber tip.

Post 3: Stock Carb Remov­al and Install­a­tion

When I first got my 650, I had a dif­fi­cult time get­ting the carb set out and had to resort to
remov­ing the left man­i­fold to provide enough clear­ance. I’ve since learned a few tricks
and now the remov­al and install­a­tion is much easi­er.
First, you’ll want to remove the cam chain adjuster cap. This will give you more room
below the carbs. Next, and this is the big­gie, remove the but­ter­fly shaped brack­et
between the carb tops (if you have one). This brack­et stops the carb bank from mov­ing
far enough to the rear to clear the intake man­i­folds. With it removed, the carbs will pull
back far enough to clear the man­i­folds and you won’t have to pull one. Once clear, you
can work the set out the left side. It will take a little rolling and twist­ing, but they will
come out.
Now for get­ting the carbs in and out of the man­i­folds. For remov­al, first you’ll want to
lube the boots with WD40 or sil­ic­one spray. Rock the carb bank up and spray some
lube in the bot­tom gap, rock it down and lube the top gap. Now rock them up and down
to dis­trib­ute the lube. Once that’s done, it’s time to pull ’em. Use the same up and down
rock­ing motion as you pull back on the carbs and they should work their way right out.
For install, lube the inside of the boots and the out­side of the carbs. Then use the same
rock­ing motion as you push them in. You’ll feel a bit of a “pop” as they fully seat in. The
WD or sil­ic­one will work here for lube but Race Tech makes a seal grease (for oil seals)
that is very rub­ber “friendly”. White lith­i­um grease can also be used (just a thin smear -
don’t goop it on).
If you’re doing this in the dead of winter in an unheated gar­age or the boots are really
old and hard, you might want to heat them up a bit first with a hair dry­er. I know my
carbs cer­tainly popped into my new soft man­i­folds easi­er than they pulled out of the old
hard ones.
A note on the man­i­fold clamps. These need to be tightened fully so the 2 clamp halves
touch, espe­cially on new soft man­i­folds. These old bikes, even in a good state of tune,
some­times spit back or back­fire through the carbs, usu­ally on start-ups. If the clamps
aren’t tightened fully, the carb can pop out of the man­i­fold. If the head of the ori­gin­al
phil­lips clamp screw is buggered up at all, you may not be able to fully tight­en the clamp
so I sug­gest switch­ing to allens. While the ori­gin­al phil­lips screw is longer, an M5 x 16
allen is plenty long enough for use here.


Post 4: Slides and Floats

Slide dia­phragm func­tion can be tested as fol­lows. If carb top screws were removed
above, rein­stall them. Then look at the backs of the car­bur­etors. At the top of each
intake bell there is a curved hole which opens into the cham­ber below the dia­phragm,
expos­ing it to atmo­sphere. If you raise a slide, cov­er the port firmly with a thumb, then
release the slide, it should fall very slowly. If it falls quickly, the dia­phragm is breached,
and the slide/​diaphragm assembly must be replaced. Mem­ber don­mur­ray has noted
that there is a pas­sage from the dia­phragm cham­ber to the enrich­er cham­ber which is
opened if the enrich­er cov­er or enrich­er plun­ger is removed. The pas­sage must be
covered to test dia­phragm integ­rity.
Remove the carb tops and springs and push the slides up, remov­ing the needle
retain­ers and needles with them. Free the dia­phragm gently. Use marked plastic bags
to keep the slides paired to their car­bur­etors. Remove retain­ers and needles. Exam­ine
each dia­phragm visu­ally for pin­holes against a bright light.
Note that in addi­tion to the hole for the needle, there is a second hole in each slide.
When the throttle is opened, air passing though the ven­turi (cent­ral part of the
car­bur­etor) into the engine sweeps past this hole, cre­at­ing neg­at­ive pres­sure in the
upper part of the dia­phragm cham­ber, which causes the slide to lift. The hole must be
clear. If it is obstruc­ted, clear it with com­pressed air, prob­ing with a wire if neces­sary.
Bag the marked slides and set them aside to pre­vent dam­age or con­tact with solvents.
To rein­stall, note that each dia­phragm has a “tab” that fits a match­ing recess in the carb
body. Be sure to align the dia­phragms cor­rectly.
The floats hang from a pin passing through two posts cast into the carb body. Care
must be taken not to dam­age the posts. A broken post can be repaired by care­fully
drilling small match­ing holes in the post and the carb body and pin­ning the pieces
togeth­er, using JB Weld as an adhes­ive; but this is a touchy pro­ced­ure, and dam­age is
entirely unne­ces­sary. Don’t try to beat the pins out!
BS38 carbs (70−79) use straight pins that fit flush with the sides of the posts. The pins
should push out. If they do not, apply a bit of spray-type carb clean­er and try to free
them. As a last resort, set a small nut against one post to relieve the end of the pin,
drive a tack through a thin piece of car­pen­ter­’s shim so that the point pro­trudes, and use
a small C‑clamp to gently press out the pin. As soon as you have move­ment, apply
more solvent and extract the pin with pli­ers. Remove the floats and float needles. Keep
float needles paired to their car­bur­etors. If needle seats are var­nished, spray with carb
clean­er and clean with Q‑tips. The seat is threaded; turn CCW to remove. If needles or
seats show wear or cor­ro­sion, replace them.
BS34 carbs (80−83) use pins with flat heads and ends that pro­trude slightly from the
posts. They are snap-fits, and must be pressed out. Relieve the head of the pin with a
nut, press the pro­trud­ing end flush with the post with a small C‑clamp, and work the pin
free by pry­ing gently under the head with a screw­driver until it can be gripped with pli­ers
and pulled out. The BS34 float needle is retained by a screwed-down tab, and it is
Viton-tipped; avoid con­tact with solvents. The BS34 seat is push-in type, sealed and
secured by an O‑ring. There is a fuel screen above the needle seat in many BS34
carbs. Be sure it’s clear. Clean with solvent and com­pressed air. Again, keep seats and
needles paired.
If the carbs have been in recent ser­vice, shake the floats. If you feel fuel slosh­ing in
them, replace­ment is the best course, although brass floats can be soldered. If the
carbs have been out of ser­vice, put the floats in a con­tain­er of gas with a lid tight
enough to push them below the level of the fuel and let them sit overnight, then inspect.
Float level is adjus­ted by mov­ing the tang that con­tacts the float needle. Floats must be
level (same read­ing on each side); this is done by adjust­ing the bridge between them.
Carbs must be adjus­ted to the same float level.
If the float bowls are fit­ted with drain plugs, fuel level can be inspec­ted dir­ectly by fit­ting
a piece of clear hose to a plug and insert­ing the plug in the float bowl. Some float bowls
are fit­ted with drain valves opened by a screw, with a hose nipple on the out­let. To read
fuel level, attach the sight tube to the out­let and set vacu­um pet­cocks, if fit­ted, on
“Prime,” and open the screw until fuel flows into the sight tube. With the motor­cycle
level, hold the sight tube upright beside the car­bur­etor body. Fuel should rise in the tube
no high­er than the lower lip of the carb body where it over­laps the float bowl, and no
lower than 2 mm. below it.
A second pro­ced­ure is to read float height. This is meas­ured from the sur­face on the
carb body on which the gas­ket seats to the top of the float, with the gas­ket removed. If
the gas­ket can­not be removed without dam­age, reduce the spe­cific­a­tion giv­en by 1 mm.
and meas­ure from the gas­ket. Hold­ing the car­bur­etor with the floats up, lower the floats
gently onto the float needle, tak­ing care not to com­press the float needle spring. Since
the read­ing is taken with the carb upside down, rais­ing the float level will lower fuel
level, and con­versely.
All BS38 carbs are set at 24 mm. +/- 1 mm.
For early BS34 carbs with brass floats, a float height of 27.3 mm. +/-1 mm. is spe­cified.
For later units with plastic floats, 22 mm. +/-1mm. is giv­en. The usu­al recom­mend­a­tion
is to use the spe­cific­a­tion giv­en for the float type. All BS34 car­bur­etors have a drain
valve at the bot­tom of the float bowl (see above). Fuel should rise no high­er than the
low­est point of the flange on the car­bur­etor body, and no lower than 2 mm. below that

Addendum 3/​9/​2009

Mem­ber “stevesemti” has iden­ti­fied anoth­er poten­tial float sys­tem defect. In pre-’78
carbs, the float cham­ber is ven­ted at the bot­tom of the float bowl through a brass pipe
seated in the float bowl, which opens above the cor­rect fuel level. It pre­vents vacu­um
from form­ing as fuel level drops and drains off excess fuel in the event of over­flow.
Steve traced a fuel leak through the vent (usu­ally caused by float level or float valve
prob­lems) to the seat of the vent pipe (below fuel level in the float bowl). He reports that
attemp­ted repair with JB Weld slowed the leak but did­n’t stop it. Any fuel leak is a
dan­ger­ous con­di­tion. It might be pos­sible to use a micro torch to sweat the pipe to the
seat with solder. Fail­ing that, the float bowl should be replaced. Thanks for the head­sup,

Post 5: Clean­ing and Adjust­ment, Pilot Cir­cuit

1. Clean­ing
The most import­ant thing to know about carb clean­ing is how to avoid it in future once
you have it done. Sintered or paper in-line fuel fil­ters will keep debris out of the
car­bur­etors. If the bike will not be run for awhile, drain­ing the carbs will pre­vent var­nish
from accu­mu­lat­ing. For exten­ded stor­age, drain the carbs, lift the slides, stick a few
silica gel cyl­in­ders from an aspir­in con­tain­er under each slide and in the air intake
throat, and seal, to pre­vent con­dens­a­tion and cor­ro­sion.
The pilot cir­cuit fuels the engine in low-speed oper­a­tion, when the slide is lowered.
On the back of the BS38 (70−79) car­bur­etor, you will see two small holes in the sides of
the intake bell (see Appendix, item 6). One is marked with a cast “M” on most
car­bur­etors This is the main cir­cuit air jet, which feeds air to the needle jet of the main
cir­cuit. The unmarked hole is the pilot cir­cuit air jet, which feeds air to the pilot cir­cuit. In
BS 38 car­bur­etors, both of these jets are fixed; noth­ing is remov­able.
In the back of the BS34 (80−83) you will find four round holes (see Appendix, item 6).
The upper pair (below curved dia­phragm cham­ber vent) vent the carb body above the
floats. Below those are the air jets. The pilot air jet is remov­able, and the main air jet is
The BS38 pilot jet seats inside the float bowl. It is the small brass fit­ting with a slot­ted
head. Above the pilot jet in the BS38 there is a recessed area in the float bowl cast­ing,
and above that, a small hole in the car­bur­etor body. This is where the pilot air pas­sage
emerges after run­ning through a tubu­lar cast­ing. On the oth­er side of cen­ter, the main
air jet feeds into a sim­il­ar hole and cast­ing. In BS34 carbs the pilot jet is under a rub­ber
plug at the end of the smal­ler ver­tic­al tubu­lar cast­ing in the car­bur­etor body; the plug
must seal cor­rectly.
Use a screw­driver or bit that com­pletely fills the slot of the jet (see Sec­tion I). If the jet
area is fairly clean and uncor­roded, one sharp push-and-twist should free the jet. If jets
are fuel var­nished, prep them for remov­al with solvent. The BS38 float bowls can be
immersed in Chem-Dip or sim­il­ar solvent. In the BS34, spray-in solvent can be applied
to the jet area and fol­lowed by com­pressed air until var­nish breaks up–never immerse
carb bod­ies. Apply a bit of pen­et­rat­ing oil and allow it to soak in. Then seat the
work­piece firmly in a pad­ded vise or clamp, and remove the jet. Use a sharp push-and
twist motion to break the jet loose. Light taps on the screw­driver will often break a
stub­born jet free. If the slot deforms, use a screw­driver with a thick­er blade or reg­rind
the one you’re using.
Once the jets are removed, check to see that the cor­rect series is in place. From 1970-
1975 (XS1 through XS650B), the BS30/​96 pilot jet was used. In this jet, the tight­est
restric­tion (meter­ing ori­fice) is loc­ated toward the barbed end of the jet, oppos­ite the
threaded end. The VM22/​210 jet was used from 1976–1979 XS650C-XS650F). In this
series, the meter­ing ori­fice is loc­ated toward the threaded end of the jet. Either series
will thread into the carb body, and install­a­tion of incor­rect jets is com­mon. Many OEM
jets were pro­duced with no bleed holes in the shank, while all after­mar­ket pilot jets have
them; this dif­fer­ence is not sig­ni­fic­ant. See Appendix items 1., 4., and 7. for pilot jet
types and stock sizes.
Soak the jets in Chem-Dip or sim­il­ar immer­sion-type solvent, and blow them clean with
com­pressed air. Blow aer­o­sol carb clean­er through the air jets and fol­low with
com­pressed air, and do the same in the pilot jet seat areas.
On BS38 car­bur­etors, the mix­ture screw is a small brass fit­ting with slot­ted head,located
on the out­ward side of each car­bur­etor, for­ward of the slide tower (see Appendix, item
9). Remove the screws and the springs below them and clean with solvent, and blow
out the seat areas. Carbs fit­ted to 1970–1975 (XS1-XS650B) require the 256 series
screw; it is short­er than oth­ers, and threaded all the way to the slot. The 584 series was
used on carbs fit­ted in 1976 and 1977 (XS650C and D); it is longer, with a relieved area
between the slot and the threads. The screw used in carbs fit­ted in 1978 and 1979 is
the longest. It is stepped, has a recessed area past the slot, and is sealed by an O‑ring.
The screws can­nnot be inter­changed. Some fuel screws were fit­ted with plastic caps to
restrict range of adjust­ment. Remove the caps and dis­card them.
The fuel screw in the BS34 car­bur­etor is recessed in a ver­tic­al tubu­lar cast­ing on the
carb body toward the cen­ter line of each carb, for­ward of the slide tower (see Appendix
item 10). Unless it has been removed, the screw is covered with a thin brass cap, which
must be removed for clean­ing and adjust­ment. Pierce the cap with a drill, being care­ful
not to dam­age the cast­ing or the screw; just pen­et­rate the cap. Some­times a sheet
met­al screw will hold firmly enough to grip the cap while pli­ers are used to extract it
(don’t run the screw in deeply; avoid dam­age to the mix­ture screw beneath). Some­times
it can be pried out after drilling. Remove the mix­ture screw, with the spring. A rub­ber Oring
is at the bot­tom of the mix­ture screw seat, with a met­al wash­er on top of it. Remove
these with a probe. When new, the wash­er is some­what flattened; this does not indic­ate
defect. Be sure that it’s flex­ible and has no nicks or cracks.
If fuel screws show signs of dam­age, wear or cor­ro­sion pits, replace them.
In the for­ward end of the car­bur­etor bore, you’ll find a num­ber of holes (see Appendix,
item 11). In front of the throttle plate (but­ter­fly), you’ll find either one or two large holes
and one small hole. One of the large holes draws mix­ture from the enrich­er, and the
oth­er will either be blind or open into the syn­chron­izer attach­ment port if the carb is so
equipped. Do not apply solvent without remov­ing the choke plun­ger (see enrich­er
sec­tion). The small hole draws mix­ture from the fuel screw. Blow aer­o­sol carb clean­er
into the hole until it flows freely from the fuel screw area. Then block the hole with a rag,
insert the tube from a can of carb clean­er in the fuel screw seat area, seal­ing with a rag,
and blow clean­er in until it flows freely from the hole in the carb body that sits above the
pilot jet (BS38) or the pilot jet cast­ing (BS34). Fol­low with com­pressed air.
Behind the throttle plate are more small holes, 2 in BS38’s, a cluster of 3 in BS34’s (see
Appendix, item 11). In early BS38 carbs these are fed by the bypass pipe, a brass tube
between the slide and the throttle plate in the middle of the bore. Find its source in the
car­bur­etor body and shoot with carb clean­er and air until clear.
In later BS38 carbs and all BS34’s, the bypass nozzles are fed from the fuel screw
pas­sage, and no tube is present. Seal the tube on a can of carb clean­er in the fuel
screw hole with a rag or drilled plug, cov­er the small nozzle in front of the throttle plate
with anoth­er rag and hold it in place with your thumb, hold the throttle plate open, and
spray until clean­er emerges from the nozzles. Fol­low up with com­pressed air.
In badly cor­roded carbs it may be neces­sary to probe pas­sages with a wire to clear
them. This should be resor­ted to only as a last-ditch meas­ure and avoided if at all
pos­sible; it’s easy to block, enlarge or deform the pas­sages, and if this occurs, grief will
fol­low and require a new carb body for cor­rec­tion.
2. Adjust­ment

Before attempt­ing to adjust fuel screw set­tings, ensure that you are not com­pens­at­ing
for incor­rect set­tings else­where. Inspect and if neces­sary adjust cam chain ten­sion,
valve lash, and igni­tion tim­ing, in that order.
Turn­ing the fuel screw clock­wise (in) leans the mix­ture, turn­ing coun­ter­clock­wise (out)
richens. Adjust­ment of fuel screws is best per­formed using the “dead cyl­in­der” meth­od.
Turn the fuel screws in until they bot­tom lightly (over­tight­en­ing will dam­age the screw
and/​or seat), and turn them out to one of the fol­low­ing set­tings.
XS1, XS1B: 1 turn
XS2, TX650, TX650A: 0.75 turns, XS650B: 0.75 turns
XS650 C, D: 1.5 turns
XS650E, F: 2.25 turns
BS34, all: 3 turns
Warm the engine to oper­at­ing tem­per­at­ure, then raise the idle to around 1500 rpm. On
bikes with break­er point igni­tions, pull off a spark­plug cap. Elec­tron­ic igni­tions can be
dam­aged by oper­a­tion with an ungroun­ded igni­tion wire. To pre­vent that, shut down the
engine, attach a spare spark­plug to a cap, ground the plug solidly on the engine, and
restart. Adjust the throttle stop to hold low­est steady idle, then move the fuel screw 14
turn each way, seek­ing highest idle. When you find the dir­ec­tion of improve­ment, set the
screw 18 turn in that dir­ec­tion from your start­ing point and again move it 14 turn each
way. As idle rises, lower it with the throttle stop, as changes are easi­est to detect at low
engine speeds. Set the screw at the inmost (lean­est) pos­i­tion that yields highest idle
speed. (And yes, I know; some own­ers advoc­ate find­ing the point where idle drops due
to lean­ness, then the point where it drops due to rich­ness, and set­ting the screw at the
cen­ter point. I do not con­cur.) Repeat for the oth­er cyl­in­der.
Note: this pro­cess will go very quickly after prac­tice but can take awhile the first few
times through. A big fan and some cool­ing breaks will do your engine (and maybe
your­self) a lot of good.

Post 6: Main Cir­cuit, Enrich­er (Choke), and Syn­chron­iz­ing

Main Cir­cuit: Clean­ing
The main cir­cuit has three com­pon­ents. The jet needle is secured in the slide by a clip
and a plastic retain­er held down by the slide spring (70−77), a snap ring (78−79), or a
screwed down plate (BS34, 80–83). It hangs into the needle jet; as the needle rises with
the slide, more fuel is drawn up through the needle jet. The main jet meters fuel flow
into the needle jet. In the BS38, the main jet threads into the float bowl. Remove it by
remov­ing the float bowl plug and insert­ing a tight-fit­ting screw­driver or screw­driver bit. If
it is badly var­nished, apply solvent to free it. In the BS34, the main jet threads into the
base of the needle jet. Remove, soak in solvent, and blow out with com­pressed air.
The BS38 needle jet is the large per­for­ated brass tube that hangs down from the middle
of the car­bur­etor body. It is secured by an O‑ring. Using solvent to free it if neces­sary,
pull it free of the carb, soak, and fol­low with com­pressed air. Avoid mar­ring or deform­ing
the needle jet with tools. Replace using a new O‑ring. The BS34 needle jet is enclosed
by the carb body and is installed and removed through the ven­turi. To remove it, install
a main jet and push the needle jet up; if neces­sary, tap it gently with a plastic drift until
the needle jet comes free, using solvent and pen­et­rat­ing oil as needed. In reas­sembly,
the BS34 needle jet is aligned by a pin in the carb body which matches a groove in the
jet. Be sure they are lined up cor­rectly before try­ing to seat the jet.
Needles are cleaned by wip­ing with solvent. Do not sand or remove mater­i­al.
Inspect the needle jets and needles for wear or cor­ro­sion dam­age. If worn or cor­roded,
replace them.
Enrich­ers (Chokes): Clean­ing
The BS-series car­bur­etor uses enrich­er valves to richen the fuel mix­ture for start­ing. In
the BS38, the valve is moun­ted ver­tic­ally and is opened by a fork on a rotat­ing shaft
which lifts the enrich­er plun­ger from its seat, admit­ting extra fuel through one of the
large nozzles in front of the throttle plate (see pilot sec­tion, above). The enrich­er
cham­ber is covered by a remov­able plate, secured by three screws. In the BS34 the
plun­ger is moun­ted hori­zont­ally in a fixed cast­ing and the fork is moun­ted on a push-pull
rod. In both, plun­gers are secured by a nut under a seal. With the plun­ger lif­ted, back
the nut off. The plun­ger can then be com­pressed against its spring, giv­ing enough play
to slip the plun­ger top away from the fork and remove it. Some mod­els have viton tips at
the foot of the plun­ger. Be sure it is intact; if it shows nicks or cracks, replace the
In both car­bur­etors, fuel is fed to the enrich­er through a brass pipe in the for­ward edge
of the carb body, which hangs down into a tubu­lar cast­ing in the float bowl (see
Appendix, item 5). The cast­ing has a port open­ing into the float bowl. Clear it with carb
clean­er and com­pressed air. To clean the fuel pipe, cov­er the enrich­er nozzle in front of
the throttle plate tightly with a rag, insert the tube from a can of carb clean­er into the
enrich­er valve cham­ber, seal­ing with a rag, and spray until clean­er emerges from the
pipe. Spray carb clean­er into the intake nozzle until it flows from the enrich­er cham­ber.
Fol­low with com­pressed air.
The throttles of the two car­bur­etors need be adjus­ted so that they open togeth­er. As in
all carb adjust­ment, make sure that cam chain adjust­ment, valve lash set­tings and
igni­tion tim­ing are cor­rect first. Pro­ced­ures dif­fer for early and late car­bur­etors.
Early carbs (70−75) have sep­ar­ate throttle cables and sep­ar­ate throttle stop adjus­tors on
the carb cable perches. They also lack vacu­um barbs on the mount­ing boots and
vacu­um ports in the carb bod­ies. Vacu­um barbs can be ordered from 650 Cent­ral and
installed in the boots. From the TX650A (74) up, boots for 78–79 mod­els with vacu­um
barbs can be installed. Barbs are covered with blind plugs dur­ing nor­mal oper­a­tion or
attached to vacu­um pet­cocks on machines so equipped.
If synch. is badly out of adjust­ment due to full dis­as­sembly of the carbs, ran­dom
tweak­ing, etc., remove the slides and sight through the ven­turi at a light source,
adjust­ing the throttle plates for equal open­ings; this should be close enough to start the
machine and pro­ceed with adjust­ment. On early machines the throttle stop (“idle”)
screws (see Appendix, item 12) are used to adjust the throttle plates. Later car­bur­etors
use a syn­chron­iz­ing screw, loc­ated between the car­bur­etors (see Appendix, item 12).
Turn­ing the screw clock­wise opens the throttle plate on the right hand car­bur­etor;
turn­ing coun­ter­clock­wise closes it.
Early machines that lack vacu­um barbs and vacu­um ports in the carb bod­ies can be
synched by the “dead cyl­in­der” meth­od. Idle on both cyl­in­ders is stepped up until high
enough for the engine to run on one cyl­in­der at a time. A plug cap is removed, and the
throttle stop on the run­ning cyl­in­der is backed off just to the point that the engine dies;
then the same pro­ced­ure is applied to the oth­er cyl­in­der. Mov­ing both screws the same
amount, idle is then set between 1,000 and 1,200 rpm. For a num­ber of reas­ons this
meth­od is prone to error, and bet­ter res­ults might be achieved by using a tun­ing
tacho­met­er to equal­ize low­est stable rpm between the two cyl­in­ders. (Note: if an
elec­tron­ic igni­tion is fit­ted, warm the engine and shut down before remov­ing a plug cap,
fit a spare plug in the cap and ground it solidly on the engine, and restart on one
On 76 and 77 mod­els without vacu­um barb boots, vacu­um fit­tings are attached in ports
on the out­side of each car­bur­etor dir­ectly above the throttle shaft (see Appendix, item
9). These are present on 78 and 79 mod­els as well. The ports are plugged by brass
screws with slot­ted heads. Remove the plugs and insert cor­rect threaded vacu­um
fit­tings; on the left side carb the fit­ting must some­times be ground down to clear the
throttle shaft return spring.
Warm the engine to oper­at­ing tem­per­at­ure. Attach a vacu­um gauge (damp until needles
still pulse slightly) or flu­id mano­met­er to the vacu­um barbs or carb body fit­tings; if
vacu­um barbs are used, set vacu­um pet­cocks on “Prime.” It is pos­sible to reach the
syn­chron­iz­ing screw between the car­bur­etors with a long screw­driver; if this is dif­fi­cult
for you, the tank can be removed and a remote fuel sup­ply can be used (see “Tools”).
Open the throttle plate on the high vacu­um side and close the one on the low vacu­um
side until read­ings are even. Very small move­ments of the screw pro­duce changes; you
only need to nudge it slightly to get res­ults.
If you are using a flu­id mano­met­er (mer­cury sticks or ATF “yard­stick” type), avoid
rev­ving the engine; this can cause mano­met­er flu­id to be sucked into the engine. If one
side starts to climb rap­idly, shut down the engine quickly and make a small adjust­ment,
and con­tin­ue shut­ting down and restart­ing until read­ings are stable enough to let the
engine run while you make fine adjust­ments.

Post 7: Seal­ing Air Leaks

Loc­at­ing Air Leaks
Air leak­ing into the intake stream leans the fuel/​air mix­ture. Symp­toms include pop­ping
in the exhaust under engine brak­ing and excess­ively high idle once the engine is fully
warm. To check for air leaks, spray brief bursts of carb clean­er on carb mount joints and
throttle shaft ends. When the spray hits an air leak, the idle will rise or fall; allow a
second or so for response.
Mount­ing Boots.
If a leak is found at the back of the mount, tight­en the clamp and retest. If unsuc­cess­ful,
remove the boot. If a leak is found between the boot and the engine, check torque on
the mount­ing screws, but don’t over­tight­en (at best, over­tight­en­ing only deforms the
flanges under the screws, leav­ing the rest of the seal­ing sur­face unaf­fected). If
unsuc­cess­ful, remove the boot.
Exam­ine the boots for cracks, cuts and oth­er dam­age, and for flat­ness of the seal­ing
sur­face. If they have been over­tightened and warped (see above), it is some­times
pos­sible to relieve the area under the screws and get them to seal, using fresh gas­kets
and seal­ant. Exam­ine the screw holes in the cyl­in­der head; these some­times become
extruded from over­tight­en­ing and must be flattened. If this is the case, seal the intake
port to keep grit and debris out; then use a flat file (one you’ve checked for flat­ness) to
care­fully level off the intake flange area and fol­low up with 400 grit paper.
Throttle Shaft Seals
BS38 (70−79) and BS34 (80−83) throttle shafts have a rub­ber seal at each end. Before
attempt­ing to replace the seals, remove the throttle shaft springs and check the shaft for
play. If it’s wobbly, new seals may work for awhile, but the pivot area in the throttle body
is worn. Replace­ment is cheap­er, but motor​cyc​le​carbs​.com reman­u­fac­tures to spec.
Mike’s XS lists seals for the BS38 but excludes the BS34 from the applic­a­tion list. For
the pur­poses of this art­icle, I dis­mantled a BS34 from a 1982 machine. Com­par­ing its
seals to a new one sold by Mike’s for the BS38 showed no vis­ible differences–same
seal type, depth, and dia­met­ers. The reas­on for the exclu­sion may simply be that for
some inscrut­able reas­on the seal was not shown in XS650 parts books from 1980 on,
and Mike’s has fol­lowed that lead.
To remove the throttle shaft for seal replace­ment, the throttle plate must be removed. If
you open a throttle and look at the back of the throttle shaft, you’ll notice that the tips of
the screws pro­trud­ing from it are cross-hatched. They’ve been staked–that is, deformed
slightly to pre­vent them from back­ing out, enter­ing the intake port, and tear­ing up more
stuff than you even want to think about just now. They’re brass, so it’s very easy to
des­troy the head, and remov­al is usu­ally trouble­some.
(Edit, 10/​01/​08: I’ve noticed that Mike’s XS has changed
the applic­a­tion list to include the BS34. You’re wel­come, Mr. Lalonde.)
Your chances are improved by using the right tool and con­trolling the work­piece. First,
note the ori­ent­a­tion of the throttle plate, and mark the shaft for its exact pos­i­tion. If
you’re rebuild­ing more than one carb, keep the throttle plates paired to them. Set the
carb spig­ot-up in a care­fully pad­ded vice, with the slide removed and the back of the
throttle shaft supported–a wood or plastic screw­driver handle minus the shaft or a piece
of large wooden rod works well. Choose a screw­driver with a pre­cise fit to the screws,
give it a light but firm tap to seat it, and use both hands to apply a short, sharp push­and-twist.
This will usu­ally break the screw loose. If the screw does not break free, try
heat­ing the area with a hair dry­er or sol­der­ing iron (not a torch or high-temp heat gun!)
and quench­ing with pen­et­rat­ing oil from a spray can. Back it out two-handed, apply­ing
down­ward force as you turn, until the staked brass cleans itself up in the steel threads
of the shaft and turns eas­ily. If the screws must be drilled out, bear in mind that a trip to
the machine shop will be cheap­er than new car­bur­etors.
Mem­ber Fry pos­ted an excel­lent tip on remov­ing the screws. After care­fully drilling the
screw heads and remov­ing throttle springs and stops so that the plates would rotate
fully, he was able to grip the staked tips of the screws with needlen­ose pli­ers and back
them out. Thanks, Fry!
When you reas­semble, use blue Loc­tite and res­take the screws with lock­ing pli­ers
(Visegrips) or water­pump pli­ers (Chan­nel Locks).

Post 8: Tun­ing for Modi­fic­a­tions

There are sev­er­al jet­ting guidelines out there already, most not­ably the tips on the
MikesXS site and the “Min­ton Mods” art­icle. In this sec­tion, I’ll attempt to expand on
those, plug any holes they’ve left, and cor­rect any inac­curacies I’ve found.
Some import­ant notes up front. Every one of these 650s seems to respond dif­fer­ently to
mods and the jet­ting changes required for them. You must base your jet­ting on what
was stock for your year carb set, not the year of the bike. Many of these bikes have had
dif­fer­ent year carb sets swapped onto them. The jet­ting specs were changed many
times over the mod­el run. Also check the jet­ting you have as it may have already been
fooled with. CV carbs are very easy to over-jet. The slide lifts by vacu­um and only as
much as the engine demands or needs. For this reas­on, you can have way too big of a
main and the bike will still seem to run halfway decent. The slide isn’t lift­ing fully so that
large main isn’t flow­ing at its max rate.
Now for some cri­tique of the old stand­ards. The “Min­ton Mods” deals with a ’78 Spe­cial
so his jet­ting guidelines really only apply to the ’78-’79 carb sets. Also, he makes no
men­tion of chan­ging pilot jets, which these bikes usu­ally need, espe­cially if you drop
your needle all the way to its low­est pos­i­tion like he did. I guess he did­n’t mind that huge
flat spot he must have cre­ated just off idle .
The MikesXS Tips for the BS38s seem to mir­ror Joe Min­ton’s as far as main jet size
goes. A 140 to 145 main is usu­ally fine for a ’78-’79 carb set which came with a 135 as
stock but would prob­ably be too rich for the earli­er carb sets. For instance, the ’76-’77
carb sets had a 122.5 main as stock. I doubt you’d get a minorly mod­ded ’76-’77 mod­el
to run right with 140 to 145 mains. That’s an increase of 7 to 9 sizes which is a big jump
for any bike. Mike’s recom­mend­a­tions for the BS34s seem pretty good and I would
fol­low them to start. Once again, you may not need as big a main as he sug­gests (only
test­ing will tell) so don’t feel you’re doing some­thing wrong if you can­’t run that large of a
OK, let’s get star­ted, shall we? First, you must real­ize that the 3 main cir­cuits of the carb
— the slow speed or idle (mix screw and pilot jet), midrange (slide, slide needle, and
needle jet), and the high speed or main (main jet) — all over­lap the cir­cuit next to them
and changes made to one will slightly effect the oth­er. In the case of the midrange,
changes will effect both the upper part of the slow speed cir­cuit and the lower part of the
main cir­cuit. You are going to have to buy a few jet sizes and will end up with extras
when you’re done, there’s no way around this. Maybe they’ll work in the next 650 you
get, though.
Here’s the simple bare bones routine which I’ll expand upon below. Increase your main
jet size until you incur break-up or stum­bling in the upper midrange. Lower the needle to
com­pensate. The lower (lean­er) needle set­ting will lean the upper low speed cir­cuit as
well, many times cre­at­ing a flat spot just off idle, and prob­ably require a lar­ger pilot jet.
Start by dial­ing in the mains. For one mod on the 38s like fil­ters or an exhaust change, I
would go up 1 size, maybe 2 at the most. For the 34s, it’s been repor­ted that 1 size up
improves a totally stock bike, so with any mods, I would start at least 2 sizes up. Test
your jet change by run­ning the bike through the midrange, say from about 3K, up to
near red­line. You must use abso­lute full throttle for at least some of your test runs or the
glitches may not show up. This is because of the vacu­um lif­ted slide. It will mask over
minor faults if run easy. Do some easy or nor­mal wind-up tests as well, in fact, try them
first. Major jet­ting flaws will show with them but the minor ones will only show, as I said,
under full throttle applic­a­tions. If the bike pulls clean through this RPM range with no flat
spots (lean) or no stum­bling or break-up (rich), go up a main jet size. If you do
encounter a giant flat spot where the bike just kinda dies, then takes off, that’s lean and
you could prob­ably jump up 2 sizes. Keep increas­ing the mains until you start get­ting
break-up or stum­bling in the upper midrange, say from 4 to 5K. This indic­ates your
lar­ger main is bleed­ing over and over-richen­ing the upper midrange. This usu­ally occurs
before break-up in the main cir­cuit itself, like up near red­line. Break-up near red­line is a
definate indic­a­tion you’ve gone too large on your main. You’re seek­ing the size that runs
clean AND pulls the hard­est. Time to move to the midrange cir­cuit .….
As your lar­ger main bleeds over into the upper midrange, it will make it too rich.
Com­pensate for this by lower­ing the needle (raise it’s clip) in the slide. With the BS34s,
you’re kinda stuck here as their needle is fixed and not adjustable. The good news is
that they were set up so lean from the fact­ory that the bleed-over from lar­ger mains may
not cause a stumble. It may just make the mix­ture right. If you do incur a stumble in the
upper midrange on the 34s and feel you need that large of a main, your only option
would be to pur­chase the adjustable Cana­dian needles that MikesXS now offers. Once
you elim­in­ate the upper midrange stum­bling through needle adjust­ment, you can try
anoth­er size up on the mains. This usu­ally brings the stum­bling right back, though. You
can drop the needle again, but I’m not a big fan of run­ning needles in their low­est (or
highest) pos­i­tions. The carb makers choose a needle that can be run in it’s middle
pos­i­tions. This loc­ates it’s tapered por­tion best in the needle jet for fuel meter­ing. The
oth­er “gotcha” with lower­ing (lean­ing) the needle is that, while cur­ing the upper
stumbles, it makes the rest of the midrange rather flat and unre­spons­ive. For this
reas­on, I like to lower my needle the least amount pos­sible, even resort­ing to shim­ming
with a small wash­er to acheive 12 step pos­i­tions. Many times, when lower­ing the
needle, you will cre­ate a flat spot just off idle, dur­ing the idle cir­cuit to midrange cir­cuit
trans­ition. Time to move on to the idle cir­cuit .….
If you’ve cre­ated a flat spot just off idle by lower­ing your needle, you’ll need to increase
the pilot jet size. Any pilot jet change needs to be fol­lowed by a re-adjust­ment of your
mix screw. You’ve just richened the idle fuel mix so the mix screw usu­ally needs to be
turned in (leaned) slightly to com­pensate. The 34s also have an air jet you can change.
This feeds air to the pilot jet to cre­ate the fuel mix. Along with increas­ing the pilot jet
size, you can decrease this air jet to richen your idle mix on the 34s. The 38s have an
air jet as well, but it’s fixed and can­’t be changed, so it does­n’t enter the tun­ing equa­tion
with them. One size up on the pilots is the norm but you may need more. Re-set your
mix screws and test for the flat spot off idle. In 2nd or 3rd gear, slow down to near
stalling then grab a big hand­ful of throttle. You don’t need abso­lute full throttle like for
the main jet tests, just a good sized hand­ful. This will reveal if you’ve cured your flat spot
and, if you’ve gone up sev­er­al sizes and get some stum­bling, that you’ve gone too big
on the pilot jet size. Once you have the pilot jet size nar­rowed down, there’s anoth­er test
you can per­form. Ride around in a park­ing lot in about 2nd gear with the throttle just
barely cracked. Watch for minor sur­ging (too lean) or rough run­ning and stum­bling (too
rich). You may be able to cor­rect this with your mix screw set­ting or you might need to
change pilots again.

Post 9: Tips and Troubleshoot­ing

Once you get close on your jet­ting, changes aren’t going to have a giant effect. The
effects will be more subtle and a quick test run may not show all the flaws. You’re going
to have to run the bike that way for awhile and put some miles on it, maybe as much as
50 to 100. Dur­ing this time, run it hard and gentle, fast and slow. Basic­ally, run it under
all dif­fer­ent con­di­tions and see if any little
glitches show up.
Final Main Jet Selec­tion
Due to the nature of the CV carb and the way it can mask over not-quite-right jet­ting,
you will prob­ably find a series of con­sec­ut­ive main jet sizes (maybe 2 or 3) that all seem
to work. They’ll all pull clean through the upper RPMs but the lar­ger sizes will prob­ably
cause more upper midrange break-up. Even if you can clean the midrange up through
needle tun­ing, your main may still be too large. Remem­ber, we’re look­ing for the size
that runs clean AND pulls the hard­est. Run some tests with the size you’ve chosen.
From about 3.5 to 4K, grab a big hand­ful of throttle and run her up to near red­line. At
about 5K or so, you should feel a surge as the main starts kick­ing in. From there, the
motor should pretty much just zip right up to red­line. Note how hard it pulls and how fast
the RPMs rise. Now try the next size smal­ler main and run the same test. If it revs faster
and pulls harder, your ori­gin­al choice was too large. If it runs slower, does­n’t pull as
hard or rev as fast, then your first choice may have been right. To insure it is, now try
the size above it. This is one of the first rules of jet­ting, when you think you’re right, try
the size above and below to make sure. Fol­low this rule and, even­tu­ally you will find the
right main jet size.
The oth­er “tell” is gas mileage. After it’s all said and done, you may end up los­ing a mile
or 3 per gal­lon with your mods and lar­ger jets. If you drop 5 or 10 MPG, that’s a sign
you’ve over-jet­ted. Most of these bikes get about 50 MPG on the high­way, the ’80-on
mod­els with the lean­er BS34s can push 55 MPG. Now this is a tank­ful burnt mostly on
the open road, not around town. Loc­al around town rid­ing can give you low to mid 40s.
Take that into account when check­ing your mileage.
Needles and Needle Jets
We’re kind of stuck when it comes to these as there are no “oth­er” sizes avail­able
except what came in the vari­ous carb sets dur­ing the pro­duc­tion run. The 38s use a
totally dif­fer­ent style of needle jet than the 34s so no swap­ping is pos­sible there. The
34s all came with the same needle jet and needle. Now that MikesXS is car­ry­ing the
slightly rich­er Cana­dian ver­sions of these, that is an option. The 38s used 3 dif­fer­ent
needle jet sizes over the years. These were the lean­est Z‑2 in ’78-’79, the richest Z‑8 in
’76-’77, and an in bew­teen size, the Z‑6 in ’74-’75. If you wanted to lean the midrange so
you could run big­ger mains in the ’74-’77 carbs, then you could try the Z‑2 jet from the
’78-’79 carb sets. You could also try the Z‑6 in the ’76-’77 carbs for a slightly lean­er
The 38s up to ’77 used short­er 4 series needles while the ’78-’79 38s and the 34s run
longer 5 series ones. Due to the length dif­fer­ences, the 4 and 5 series needles can­’t be
swapped. This leaves you with swap­ping the vari­ous 4 series needles among the ’77
and older carbs or swap­ping the two 5 series ones between the ’78-’79 38s and the 34s.
I have examined and meas­ured the 2 later 4 series needles used, the 4N8 and the 4M1,
and there really isn’t much dif­fer­ence in the 2. I haven’t had the oppor­tun­ity to exam­ine
the early 4JN19 needle but from the photo on the MikesXS site, it appears very sim­il­ar
to the oth­er 2. I don’t see much to be gained by swap­ping them around although you
could try it. The 5 series needles in the later carbs, on the oth­er hand, have 2 very
dif­fer­ent tapers to them. You should notice a dif­fer­ence in the way the bike runs by
swap­ping them. Now wheth­er this will be an improve­ment or not, only exper­i­ment­ing will
tell. I don’t think I’d even both­er try­ing the fixed clip 5HX12 in the ’78-’79 38s but the
adjustable 5IX11 Cana­dian ver­sion might be a viable option. The 5O2 from the 38s may
work in the 34s as well. Again, only exper­i­ment­a­tion will tell. The only oth­er needle
options would come from oth­er CV carb sets used on vari­ous oth­er bikes of the era. I’ve
examined a few and the only viable one I’ve found so far is the 5Z1 needle from the
XS400 Yamaha. It’s the same length as the 5O2 but has a taper that more resembles
the 5HX12. I’ve run it on and off over the last couple of years in my ’78 carbs and it
seems to give bet­ter lower midrange response because it’s taper starts soon­er than the
one on the 5O2. It is fat­ter (lean­er) in it’s upper midrange area so I don’t recom­mend it
for bikes with the stock mains. You want the bleed-over from lar­ger mains to counter the
lean­er upper midrange this needle provides.
Needle Shim­ming
I men­tioned in the “Mods” sec­tion that I some­times shim my needle height. Every clip
pos­i­tion on these needles changes the height by about 1mm. There is usu­ally a small
flat wash­er either above or below the clip (depend­ing on the slide type) and this is about
a half mm thick. By acquir­ing and adding anoth­er of these wash­ers, you can achieve
what are equi­val­ent to half notch pos­i­tions, say 2 12, 3 12, etc. Why both­er with this?
Well, as I men­tioned, lean­ing the needle to cure the upper midrange stumble has it’s
draw­backs. It makes the lower part of the midrange rather flat, unre­spons­ive, and
slug­gish. Lean­ing the needle set­ting as little as needed is best. In one of my bike’s
rendi­tions, I ran the equi­val­ent of a 2 12 notch set­ting. This was the richest I could set
the needle and still cure the upper midrange stumbles caused by my lar­ger main.
On the ’77 and older carbs, the needle hard mounts into the bot­tom of the slide and is
spring loaded from above. On these, place the shim wash­er below the clip, between it
and the slide. It will raise whatever clip pos­i­tion you’re in by 12 step. On the ’78-’79
carbs, the needle is spring loaded from below. The shim wash­er needs to be above the
clip, between it and the thick plastic wash­er. This will lower whatever notch set­ting
you’re in by 12 step. As an example, say you have both a ’77 and a ’78 carb, needles
both set in the 3rd slot. Adding the shim wash­er under the ’77 needle clip would give
you the equi­val­ent of a 3 12 notch set­ting. Adding it above the clip on the ’78 carb
would put you at a 2 12 notch set­ting.
I’ve not men­tioned the BS34s because their needle is fixed. It is hard moun­ted into the
bot­tom of the slide with a small spring on top, some­what sim­il­ar to the ’77 and older
carbs. The only way you can shim it is up (rich­er) but when installing lar­ger mains, you
usu­ally need to do the oppos­ite, lower or lean the needle set­ting. If you switch to the
adjustable Cana­dian needle, then yes, you could shim it that half step by adding the
wash­er beneath the clip.
Slide/​Diaphragm Assem­blies
Obvi­ously, the 38 and 34 slide assem­blies can­’t be swapped because they’re dif­fer­ent
dia­met­ers. The 38s used 2 dif­fer­ent slide assem­blies. Carbs up to ’77 used one style
while the ’78-’79 carb sets got a redesigned slide that used a longer 5 series needle and
moun­ted it dif­fer­ently. Even though the 2 slide assem­blies will phys­ic­ally inter­change,
MikesXS claims you can­’t swap them — but maybe you can. Even though the ’78-’79
slides use a longer needle, they mount it high­er up in the slide. The part that mat­ters,
the amount of needle hanging out from the bot­tom of the slide, is near identic­al on both
the new and old slide designs. This leads me to believe that you can swap the slide
assem­blies as long as you use the needle that came in that slide design and, pos­sibly,
it’s match­ing needle jet. This may require you to juggle main and pilot jet sizes as well to
more closely mir­ror the set­tings from the carbs the slides came out of. Since the ’78-’79
slide assem­blies are not avail­able at the moment, fit­ting an earli­er slide set may be your
only option if you want to save a set of these carbs that need replace­ment dia­phragms.
Pilot Jets
As men­tioned else­where, 2 types of pilots are used in the stock carbs. While they look
the same on the out­side and will phys­ic­ally inter­change, they flow dif­fer­ently. By
study­ing the stock sizes of the 2 types that were installed over the years, you can see
that for any giv­en size, the BS30/​96 style flows less than the VM22/​210 style. The 650
needs BS30/​96 type pilots in the low to mid 40s to flow as much as VM22/​210 pilots
sized in the mid to upper 20s. The 34s used only the BS30/​96 type so I would­n’t try the
oth­ers in them (although they would prob­ably work). The 38s, on the oth­er hand, used
both styles. There were dif­fer­ences in the air feed pas­sages to the pilot jet so you must
use the type speced for your year carb set. BS38s up to ’75 used the BS30/​96 style pilot
jet, ’76-’79 carbs used the VM22/​210 style.
Anoth­er ques­tion that often comes up is the need for the air bleed holes in the sides of
the pilot jet. If you pulled the ori­gin­al pilots from your 38s, you may have found that they
had no bleed holes. They wer­en’t needed on the 38s because the pilots mount upside
down, flow in reverse, and flow only fuel or the already made fuel/​air mix — the air that
would “bleed” through the side holes is added above or below the jet. The pilots on the
34s, though, mount in the nor­mal man­ner, screwed up into the main carb body. Air is
fed to the side of the jet (again, the norm) to cre­ate the air/​fuel mix so the bleed holes
are required in the 34 pilot jets.
Port­ing Effects on Jet­ting
Will you need to re-jet after a port­ing job? Well, prob­ably some minor changes will be
needed. You’ve altered the flow after all, or should have if you did things right. Fol­low­ing
Jack­’s excel­lent posts, I just did a basic clean-up and smooth­ing on mine with a Dremel.
After pol­ish­ing and remov­ing all the lumps and bumps from the intakes, I roughed them
back up with a coarse sand­ing roll to aid fuel atom­iz­a­tion. The exhausts were left
smooth and pol­ished. I was able to go up anoth­er size on my mains but I needed to
reduce the pilots jets one size (I was up 2 over stock) and set the mix screws lean­er.
Your res­ults will prob­ably vary but some minor tweak­ing will most likely be called for.
Am I Too Rich or Too Lean?
Plug read­ings, of course, will give an idea when you’re first start­ing out. All white or all
black will indic­ate way too lean or rich. Once you start get­ting close, though, plug
read­ings won’t change that much. You’ll have to rely on more subtle signs and “seat of
the pants” feel as to how the bike is per­form­ing. You can still check your plugs but now
you’ll be look­ing for and read­ing what’s called the “smoke ring”. This is the ring of col­or
on the por­cel­ain por­tion of the plug. Ideally it should be a nar­row ring way down at the
bot­tom of the por­cel­ain, barely vis­ible without a good light source and some­times a
mag­ni­fy­ing glass. A small change may also take a few miles before it shows any­thing
on the plug.
Read­ing the “smoke ring” works best when doing full throttle chops with new plugs but I
find it very help­ful down the line to give you a gen­er­al indic­a­tion of your carb set­tings.
As an example, I knew I had my carbs set up pretty good but wanted to try a rich­er
midrange set­ting just to see what it felt like. I moved my needles one notch rich­er and
went for about a 20 mile test run. A plug check after­wards revealed that my near per­fect
“smoke ring” had grown from the nar­row band at the bot­tom to cov­er nearly 12 to 23 of
the por­cel­ain — a def­in­ite indic­a­tion that I was set too rich. For more info than you’ll
prob­ably ever need on plugs and plug read­ing, have a look at this art­icle by Gor­don
OK, here’s a list of some of the subtle and not so subtle signs you can use to
troubleshoot your jet­ting. Some of this will take an exper­i­enced ear and famili­ar­ity with
your machine. In oth­er words, if you bought your first XS650 yes­ter­day, I don’t expect
you’d have much luck jet­ting it today .
— A very rich con­di­tion usu­ally causes break-up or stum­bling which gets worse as you
apply more throttle. Just off idle, this would indic­ate too large a pilot jet or to a less­er
extent, too rich a mix screw set­ting. Try lean­ing the mix screws first but you will prob­ably
need a smal­ler pilot — that’s the usu­al cause. Through the lower midrange would usu­ally
indic­ate too rich of a needle clip set­ting. In the upper midrange can be that clip set­ting
again but is most likely too large of a main bleed­ing over — or a com­bin­a­tion of both. Up
near red­line would be, of course, too large of a main.
— A too lean con­di­tion usu­ally mani­fests itself as a flat spot, seem­ingly like you switched
the motor off and on quickly. The engine will fall flat on it’s face, like it’s run out of gas
(which, tech­nic­ally, it has) then recov­er and accel­er­ate again once through that lean
RPM range. The RPM range in which this hap­pens will basic­ally tell you which cir­cuit to
tweak first although you may need to make some changes in the cir­cuit next to it as
well. Remem­ber, they all over­lap the cir­cuit next to them to a cer­tain degree.
— There is also a lean mis­fire or stumble. This is much like the “too rich” indic­at­or but will
sound dif­fer­ently, usu­ally sharp­er pitched. This is where that exper­i­enced ear comes in.
Also make note of where you are in your jet­ting changes. If you’ve done your mods and
haven’t made any jet­ting changes yet, then it’s a pretty safe bet that your mis­fire or
stumble is one caused by a lean con­di­tion. You can only go up or down on a jet size so
you’ve got a 50–50 chance of mak­ing things bet­ter or worse. Pick a dir­ec­tion to go, step
up/​down one size and see what the res­ults are.
— Some more main jet tests .… If when rolling off from full throttle to about 34, you get
some sur­ging, that may indic­ate leaness. If you go from full throttle to about 78 and get
some break-up, that can indic­ate rich­ness. These 2 tests work bet­ter on reg­u­lar slide
carbs but may help on your CVs. One test that does work on the CVs — if you roll off
from abso­lute full throttle slightly and the bike pulls harder, that’s an indic­a­tion that your
main is too big.
— There is an inverse rela­tion­ship between main jet and needle jet size. In oth­er words,
lar­ger mains are run with smal­ler needle jets, smal­ler mains with lar­ger needle jets. This
is true for all carbs, not just these CVs. Jet up on your mains high enough and,
even­tu­ally, you’re going to need to lean the midrange out, either through needle set­tings
or a jet change (maybe both).
— Too large of a pilot jet can be indic­ated if when blip­ping the throttle, the RPMs fall back
down below your set idle speed, then recov­er. You may be able to tune this out with the
mix screws.
— Too small of a pilot can be indic­ated by an idle that “hangs” after blip­ping the throttle or
by hav­ing the motor be slow to return to idle after run­ning at high­er speeds. Once again,
mix screw set­tings may help here.
— Pop­ping from the exhaust on decel­er­a­tion usu­ally indic­ates leaness in the pilot cir­cuit.
A lar­ger pilot or rich­er set­ting on the mix screws (some­times both) is the usu­al cure.
— When you set your mix screws, you will usu­ally find a nar­row range, say a half turn or
so, where the bike idles the fast­est and smoothest. This is what we call the “sweet spot”
and your mix screw should be set some­where in that range. Exactly where is open to
some debate. This “sweet spot” should be loc­ated close to (with­in a quarter turn or so)
or right at the sug­ges­ted set­ting for your carb set. If your best set­ting is a turn or 2 away
from the fact­ory spec, that usu­ally indic­ates some prob­lems. Either your idle cir­cuit
needs clean­ing or you need a dif­fer­ent size pilot. The carb man­u­fac­tur­ers like the mix
screws set between 1 and 3 turns out. This places the tapered end of the screw in the
pas­sage­way so it meters flow the best. It also insures that if there is a ten­sion spring on
the screw, it will do it’s job and hold your set­ting for you. Spring loaded mix screws that
are set 4 or 5 turns out are no longer spring loaded and can fall out. So, if you’re out­side
that 1 to 3 turns range, you should change pilots. If you must set the screws out­side of
the “sweet spot” for the bike to run right else­where, change pilots. If you must set the
screws out­side the “sweet spot” to cure pop­ping on decel, change pilots. Set­ting the
screws to the rich side of the “sweet spot” usu­ally helps with the pop­ping but if it won’t
cure it, try the next size lar­ger pilot.


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