Ok put a 290 big bore kit with stroker crank in 2004 yz250f the thing won't stop backfiring . Runs good will idle but back fires (not pops) when letting of gas and during . Not as much though.doesn't back fire when choke is on ? The best I can get it is with a 45 pilot and 182 main . Thought these signs ment to rich so went down to 180 worse so went down to 178 not as bad as 180 not as good as 182? Try a 185? What is going on? Bike starts best with 45 pilot checked for exhaust leaks .None . Air filter is brand new . ?????? some one help

Sometimes I have heard it is the valves that can also cause this problem. I am not for sure. Most bikes pop and backfire but not a lot at all. I'd take it to a local shop and have them check it out. Well worth it

What is your air fuel mixture screw set at? Try massively oiling your air filter and turning your air fuel screw 1/2 to 1 full turn out from factory settings.

Edited by Free2Ride

Sounds like you are way too rich. You usually have to go leaner on the jetting for a big bore. Use the forum search function or the old Google and I bet you can find tons of info on it. Good luck.

I have tried air screw at all turns . Thanks guys

But y would it work good with the choke on doesn't that add gas and less air . So I would think that it needs to be richer . But I'm thinkin maby a 175 might do the trick . Or its not my jetting

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