pop'n when I let off the gas

My '01 YZ 426 started to back fire and pop when I let off the gas. Is this normal or is the valves need adjustment? :thumbsup:

I will get an occasional pop or two but it shouldnt do it every time you let off the gas. Could be valves, or running lean.

My 04 started doing that when the topend was getting tired my mechanic buddy told me as a motor gets decreased compression from righ blow-by it also pulls in less fuel causing a lean condition. I would get a leak test done on it if you don't have and exhaust leak as Gray suggested.

Didn't say that.

Do a compression test, if that is ok check for exhaust leaks, loose air boot ect..then move on to your valves and check them. When troubleshooting start small go big.

It'd be much simpler to run that process the other way 'round. Adjust the idle mix for best idle by the book first, and if that doesn't resolve the matter, check for exhaust leaks. Follow that with a valve clearance check.

Compression checks are functionally impossible with a YZF. When they're healthy, you can't kick the old ones through a full compression stroke, and the newer ones have auto decompression. Yamaha doesn't even publish compression specs for them. Even if possible, a compression test tells you very little compared to a leak down test, anyway, and as a result, cranking compression tests are pretty much obsolete.

Besides, low compression has very little to do with decel backfire as it is.

If you mounted the throttle slide plate square end up, the engine will pop and be untunnable on low rpm.

That happens to a lot of people as its very easy to make this mistake

My '07 right out of the crate would not only constantly backfire on decell, but would shoot small quick crackly flames out the exhaust (in colder temps). Richening up the pilot jet by one size made a big improvement on low speed behavior...

So not valves?

I'd def check the valves...too, i mean really, when was the last time you had a feeler guage in there?

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