Carb jetting

Yeah, I might be due to check all of those things. I haven't done that in a couple years. Yeah . . . years. Ha ha. I'm probably due.

I'm not mechanical enough to do it myself, although I'd imagine it would be a half fortune to take it somewhere to have this all done. Thoughts?
 
Now you're hitting on the crux of this forum. Seems that very few shops will take these on, or even want to. We're a thrifty (cheep) lot, and found that it's better to be the master of your domain, especially when talkin' carburetors (Huh? Wuzzat? Where's the injectors, power commander, and plug-in for my diagnostic tools?)

These things really are antique collectables. Near impossible to find parts for many of them, and then it's sub-par quality for a lot of it. To survive here, especially if it's a daily driver, it's best to know how to work on it.

Imagine if you had a '75 Vega.


And took it to the shop...
 
Yeah, makes sense. I have the manual, and I get the gist of how to set all of those things. Plus there are forums galore to help trouble shoot. My issue, is that I don't have the tools to get the job done. I have the most basic of mechanical tools.
 
There are no short cuts to maintenance. Sea Foam Is a good product. It can help keep a clean system clean, it can even clean up a certain amount of crud. But if the crud is a heavy build up the Sea Foam will loosen this crud all at once and this crud can plug things up.
If you want the bike to run well you need to read the carb guide then us it to tear down, clean, inspect what you have. If on inspection you find something bad replace just what you need. Most kits won't have much you can use.
Leo
 
Ok.... i probably looked over this about 10 times.. I need to figure our what size main and pilot jets I need... short open exhausts with pods. I cant seem to find the stock sizes.. 1982 Xs BS34 so confusing. Live in cedar rapids iowa town driving with a little highway.


sorry if that question has been asked over again.... new to all of this. thanks.
 
:agree::agree::agree:
Read the carb guide in the tech section , then print it off & read it several more times :D
make sure you are using decent air filter pods, the cheap ones can block the intake holes on the carbs & cause lots of problems. Use K&Ns or XS performance.
Im running 1977 BS38s with XS pods and one off exhausts, went up one from standard on pilot & 2 on mains, dropped needle one notch.
Not perfect but close :bike:


hey i recently bought a 1978 xs650 and i have the shorty exhausts on it, along with the XS performance pod filters. it was running too lean to begin with what i assumed were stock jets, would backfire all the way through midrange, never took it out of 2nd gear... idled fine. i went up to a 150 main, and a 32.5 pilot, plus raised my needle one notch, now the bike shoots flames when i start it up, idles at like half throttle. i dont have an rpm guage and i believe the carbs are synced as best i can get them. two questions... what did you mean by one off exhausts and does your bike run/idle well with the changes youve mde???
 
ok, i have vm34s-bike would start but die after 10 sec.- now i got it to stay running but the idle is crazy fast like 5-6k and I can't get it to slow down with idle screw. that being said here's what i got 1-1/2 shortty pipes- velocity stacks-15 or 1.5 pilots(can't see if theres a decimal) and 175 mains-air screw 1/2 turn out-idle screw 3 turns out-fresh rebuild-starts on 1st. kick and idles at (guessing)5-6k--i don't want to blow it up need advice on what to try- floats are correct at 24mm-carbs are new from tc bros
 
Get a mirror and see how far the butterflys are open, not much more than a "sliver" is needed for idle and both need to be the same. Funny things can happen to throttle cables as the carbs are installed.
 
If you have VM carbs then there are no butterflies, just cable operated slides. The idle speed screws lift the slides slightly and that's what controls the idle speed. You could have the slides installed wrong, maybe backwards or switched left carb to right carb. Is there any freeplay in the throttle cables? If not then they could be holding the slides open and causing the high RPM running.
 
If you have VM carbs then there are no butterflies, just cable operated slides. The idle speed screws lift the slides slightly and that's what controls the idle speed. You could have the slides installed wrong, maybe backwards or switched left carb to right carb. Is there any freeplay in the throttle cables? If not then they could be holding the slides open and causing the high RPM running.
yeah, I'm thinking the same thing cable has no play at all- I hate to pass blame but I had the cable made by Motion Pro with very detailed measurements and paid for it 2x, guess i'll call then
m tomorrow and give em the bad news- I mean how hard is it to make a frikin cable? rediculous
 
VM34 question. Could anyone give me a head start on tuning this brand new pair of VM34 round slides? I purchased them brand new from Niche Cycle Supply and they are SUPPOSED to be "PRETUNED" for XS650's. I got the bike running but it seems excessively rich and the venturie looks like it should be in a 2 stroke. What main, bleed, needle, venturie??? I would appreciate any input.
 
VM34 question. Could anyone give me a head start on tuning this brand new pair of VM34 round slides? I purchased them brand new from Niche Cycle Supply and they are SUPPOSED to be "PRETUNED" for XS650's. I got the bike running but it seems excessively rich and the venturie looks like it should be in a 2 stroke. What main, bleed, needle, venturie??? I would appreciate any input.
http://www.amckayltd.com/vm34-36.pdf
 
VM34 question. Could anyone give me a head start on tuning this brand new pair of VM34 round slides? I purchased them brand new from Niche Cycle Supply and they are SUPPOSED to be "PRETUNED" for XS650's. I got the bike running but it seems excessively rich and the venturie looks like it should be in a 2 stroke. What main, bleed, needle, venturie??? I would appreciate any input.
I had to order #20 jet and 190 main and it runs fine now still a bit rich but idles nice and no popping- its going to take some time but just keep changing them till you find the right set up. 3rd slot on needle also double check throttle cable!! so slides are all the way down.
 
Fastest, the venturi is the main bore of the carby, and you can't change that unless you bore it out bigger. Now guys, heads up here. There are quite a few combinations that work on these motors. Sudco likes to use rich 2.0 slides. Those force the use of lean pilot jets, usually 17.5. The factory generic slide is 2.5. With those, all other things being equal, we usually wind up with a 25 or 22.5 PJ on the XS650. Secondary effects of component changes are very strong in these carburetors, so when you read what's worked for who, you have to consider the whole jet list. Only complete lists have any practical meaning at all. The heart of the carburetor is the needle jet and needle. Some vendors like to leave the factory generic stuff in place because it cuts cost. If the generic Q2 NJ is still in place your jetting will never be right. Period.

Be aware that weak ignition can imitate a rich fuel condition. Before any attempt to fine tune carburetors, everything else should be inspected first.

One more time, here's a baseline that shouldn't require more than minor tweaks. Slide: Generic 2.5. PJ: 22.5 or 25, have both on hand. NJ: P5 or P6 (one or the other, a single step makes little difference). JN: 6F9. MJ: 180-190 (have 180, 185, an d 190 on hand, adjust range leaner for high altitudes). AJ: Remove.
 
Reply above is damn near spot on.Ya gotta buy the new needle jet and needle for the generic 34 mm Mikuni VM carbs.Minor tuning after that to dial them in for your specific engine combo of parts.And the weather.And the altitude.But those settings will get it close enough to idle and tune further without fouling plugs.Unless you have other issues-valve adjutments,cam chain adjustment,ignition problems or timing off,or low compression or oil blow by fouling plugs.
 
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