Heavy back fire when off the throttle?!? Carb? Valves?

4 replies to this topic
  • mdubpower

Posted June 27, 2010 - 05:56 PM


I just picked up a 02 YZ426F. It has a pretty rough time keeping an idle and it back fires really bad only when releasing off the throttle. The guy I got it off claims that it ran well until he pulled the carburetor and cleaned it. Obviously I take what he says with a grain of salt because I only paid $550 for it.

Should I rebuild the carburetor or does this sound more like it needs a valve adjustment? It does have an FMF pipe on it, but I don't think it should back fire this bad.

Also it takes a few kicks with the decompression on, and then with it off, it usually starts after a kick or two. Is this normal? Is it just me or does it feel like you need to be able to squat 400 pounds in order to kick this thing over, I'm use to two strokes so this is a whole new ball game for me.

Thanks in advance!

  • crf450319

Posted June 27, 2010 - 07:14 PM


Backfiring on deceleration can be caused by a lean condition, in my experience it's been a result of a cracked header. Take a look/feel at your header (backside and bottom too), it's the easiest thing you can eliminate as being the culprit.

Something else that's relativly easy to check is the valve clearance, go to the sticky at the top of this forum and have a look. There are step by step instructions and all you need are some basic tools and a $4 feeler gauge.

Good luck !

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  • grayracer513

Posted June 27, 2010 - 07:52 PM


Incorrect idle mixture or something as simple as a loose mid pipe clamp.

  • mdubpower

Posted June 28, 2010 - 08:29 AM


What should the idle screw be set at? I'm at sea level so I should assume that it would be 1.25-1.50 turns it right?

  • grayracer513

Posted June 28, 2010 - 08:37 AM


What should the idle screw be set at? I'm at sea level so I should assume that it would be 1.25-1.50 turns it right?



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