The XS650 Garage Carburetor Guide Introduction 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.
...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 ....
Post 2: TOOLS Screwdrivers. 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.
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.
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).
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.
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.
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.
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.
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.
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.
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. ©2007, , All Rights Reserved. To the best of our knowledge, the text on this page may be freely reproduced and distributed. Sale of this information is prohibited.
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.
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.
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").
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.
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.
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.
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.
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.
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.
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'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.
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.
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.
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.
Post 10: Appendix - Charts, Diagrams, and Pictures They say a picture is worth a thousand words. Since we've already written at least that, maybe this will save us having to write the 2nd thousand . 1. Stock Carb Specs (U.S.) - Here's a useful chart to help you identify your carbs, re-jet them, or choose a set for a carb swap .....
2. Basic CV Carb Operation - Here's a diagram outlining the basic systems and how the CV carb operates. Open it in a separate window and follow along as you read the text .....
3. Starter System - Here's another outlining the Starter or Choke Circuit operation .....
4. Pilot Jets - A comparison chart of the 2 types of pilots used in the stock 650 carbs .....
5. Starter Circuit - A photo of a rather ratty set of 34s showing the Starter Circuit components. The Starter jet is at the bottom of that bored hole in the lip of the float bowl and not removable. All these parts need to be clean and clear for the "choke" to operate. Also of note - if you get new float bowl gaskets, they will have 2 punch-outs along the edge. One is right over that bored hole, the other about an inch away along the same side - both need to be punched out for the choke to operate .....
6. BS34 Air Jets - Those same ratty 34s, this time illustrating their often mentioned removable Pilot air jet (removed in this photo) and the fixed Main/Midrange one. The larger holes above them are air vents for the float chamber and the slot at the top feeds the slide/diaphragm to create vacuum and make it lift .... 7.
8. "No Hole" Pilot - Compared to a standard pilot jet with bleed holes. Also note the Mikuni needle - clip positions (notches or slots) are always counted from the top down .....
9. BS38 Mix Screw Location - Also note the vacuum ports available on the 38s (arrows). A suitable threaded nipple can be installed in place of the block-off screw to enable gauge hook-up for syncing. This is only necessary if you have no vacuum barbs on your intake manifolds ('77 and older). The size is a standard M6 x 1.00 ..... 10. BS34 Mix Screw Location - Block-off cap and screw removed in this photo .....
11. Mix Screw & By-Pass Outlets - These tiny holes must all be clear for your pilot circuit to function properly. This is a BS34. The mix screw outlet on the 38s will be offset to the side slightly, by-pass holes are the same ..... 12. Idle and Sync Adjustment Screws - These are 38s but the 34s are the same. Older carb sets with no linkage would have an idle adjustment screw on each carb .....
13. BS38 Float Bowl Variations - There are internal differences in the float bowls that use the VM22/210 pilot jets as opposed to those running the BS30/96 style. This involves the air passage that feeds air to the jet. This means that the bowl will only work with the type of pilot it was designed for. Here's the VM22/210 type used from '76 on. Air is directed across the top of the bowl to the top of the jet .....
And the '75 and older BS30/96 type bowl. Air is fed down an angled passageway to the bottom of the jet ..... ©2007, , All Rights Reserved. To the best of our knowledge, the text on this page may be freely reproduced and distributed. Sale of this information is prohibited.
