I just finished my single carb setup and intake. I have a mikuni vm36 carb with 200 and 25 jets. It has a straight exhaust no baffles n3c plugs. It starts and idles very well but when given too much throttle it backfires out of the left cylinder exhaust. I'm new to the carb bikes. I put the jet setup that another thread said he used into the carb. I'm not sure how to tell if it's a lean or rich backfire or if that's even possible. Any help would be much appreciated.
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Need a little help with single carb setup
- Thread starter KP73-650
- Start date
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- carburetors engine single carb
Hi, KP73-650, welcome to the forum.
The spark plugs can tell you if/which side is rich or lean. Do you know how to read plugs?
Might find some help here: http://www.xs650.com/forum/showthread.php?t=27063
The spark plugs can tell you if/which side is rich or lean. Do you know how to read plugs?
Might find some help here: http://www.xs650.com/forum/showthread.php?t=27063
Hi KP,
modified the frame too, dintcha? What you could have done was to weld an eye into the frame downtube to put the manifold through so it's legs would be the same length.
But would that matter? Look at the intake manifold on a straight 6 Chevy, 3 different intake lengths and they run OK.
If you have points ignition, check the spark timing, and for exhaust leaks too, before deciding it's a carb problem for sure.
I've tried to do some research on how to read the plugs. I need to start with a fresh set and see what happens. Anything done to the frame was done before I got it. Everything on the bike was done like that with the exception of the single set up. What plugs do you recommend?
I've always liked NGK BP8ES, though the recommended is BP7ES. Some fellows here like Autolite 63's. Could just run the cheaper plugs for diagnosing, upgrade after it's dialed-in...
Hi KP,
not picking at you or the bike, just saying if I were going single carb I'd put an eye in the frame to allow a symmetrical manifold.
Not an original idea, my plunger frame BSA twin was like that.
What you could try at no cost is swapping the ignition around so each coil fired the opposite cylinder.
If the problem moves to the other cylinder it's spark related. If it don't, it's carburation.
If I think what you mean by an eye I don't have anyway to do that it has a tc bros hardtail and the electrical can is under the seat right behind the downtube. No offenses taken here I take all opinions into consideration
I adjusted the air mix screw on the carb and put new bp8es plugs in it and I can't seem to make it backfire anymore. Now when it put some throttle to it, it will only rev to what seems like 3000 3500ish. I don't have a tach so not real sure. But when it revs to there is acts like it's hitting a rev limiter. Any ideas?
Fndirtygreaser
XS650 Addict
The madness begins. Good thing is you just have one carb rather than 2. First I would verify what slide, jet needle , needle jet you have.
Update. I read another post on here about a guy having basically the exact same problem. I thought maybe a timing issue so I went to check my points and found out I don't have points. It's been ungraded to pma and pamco. Nice little bonus I didn't know I had. Anyway they were trouble shooting his and found that his batter had a bad cell changed the battery and problem solved. So I checked mine this morning and only had 116 cca so I charged it and checked again same 116cca. Installed new battery and bam problem solved. Thank you guys for all the help.
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