Long Rev Down and Carb Backfire

Fergie · Posted: May 14, 2009, 6:13 pm Post subject: Long Rev Down and Carb Backfire

OK, so I've made some big improvements. The camshaft is timed correctly, and the carbs are to the point where the bike will start and idle. I had the headers off last night so I see the exhaust coming out (and some flames too). When the engine is idling I've noticed that the left cylinder misfires until I get the revs to 2000rpm or above. I've tried playing with the idle mixture screw, but nothing makes a difference; lean or rich. I've also tried adjusting the carb sync, and that hasn't helped either, but I haven't put a vacuum gage on it yet to see how in sync they are. Do you guys think it's the carb sync, or the idle mixture?

Next, after the engine warms up, I'll re it up to 3000+ rpm, and then it will slowly come back down to idle (1200rpm). I've verified that the throttle is not sticking, and is springing right back to the throttle stop. Could this be a leaking carb seal?

Finally, I held the engine at higher revs, around 5000rpm for 5 seconds, and then quickly closed the throttle. When I do this, I get a small "backfire" through the carb intake. It is not loud, and you can't really notice it unless you see the small puff of smoke come through the air cleaner. I know this has been discussed before, but what causes this to happen?

Thanks for putting up with me.

Retiredgentleman · Posted: May 14, 2009, 7:06 pm Post subject: Re: Long Rev Down and Carb Backfire

Fergie;
You're having lots of problems with carbs. I'd recommend you let the bike sit for awhile, and spend some time reading and re-reading all information you can find on Mikuni CV carbs. The "Carb Guide" at 650 garage is a wealth of information, complete with diagrams and pictures.
xs650temp.proboards.co...read=10538

The past threads in the "Fuel" and "Engine" section have covered most of the problem areas you are now having.

Once you have a better understanding of how these carbs work, you'll find it much easier to sort out your carbs.

Remember, the inside of the carbs, bowls etc, must be clean enough to eat off, and all passage ways, jets must be proven fully clear.

What Happened to 650rider.com?

Yamaha xs650