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06 crf 450 afterfire/backfire HELP!!

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Hi am am new to this forum

i just bought a used 2006 crf 450 it doesn't have very many hours on it the hour meter says 40 hrs but i came from Arizona USA. i live in Canada BC my altitude is about 720. the problem is when i let go of the throttle the bike it pops and bangs (after-fire)like crazy. When i hold the throttle revving it does the same thing after-fire popping and banging??

I just checked my valve clearance my intake is 0.16mm or 0.06in (to honda specs) but the exhaust valves are at 0.20mm one is tight the other fits nice (out of spec i am in the process of getting the new shims) i have a 168 main jet and the needle clip on 5 slot i have tried changing the clip position and adjusting the pilot jet several times no luck.

Would the exhaust being out of spec clearance be the reason for the after-fire ???

Any help and advice would be nice ­čĹŹ

Thanks Aaron

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