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New piston bike won't start

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We put a new piston in my buddy's 03 450 timming marks are all good it has spark but it will not start it backfires like every 10 kicks?? Looking for some help thank u

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the cam timing is off

Edited by nesc103y

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Yep. Or maybe you reinstalled the shims in different spots so a valve might not be closing all the way as the clearance is zero'd out.

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 this thread title comes up every month. as stated above, 9 times outa 10, during re-assembly, cam timing is off a tooth or so.

Edited by MELK-MAN

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Ya, check the timing like the sticky I put up at the top of the forum. One position the cam timing can look right, turn the engine 180 degrees and its one tooth to the retard, which is what yours sounds like since its back firing etc.

 

Second question...

 

Did you have the carb apart, for a nice cleaning while you were doing the top end? If so, and the timing turns out to be ok, check the steel part of the slide. You probably got it in upside down.

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Shawn, you stole my thunder.  Cam timing is the obvious culprit. +1 on  the steel part of the slide  (hole on the down side M reading like and M not a W)  if my memory serves me correclty.  Been their done that...once!  But i'll never admit to it!  Does the cam chain have proper tension?

Edited by bikedad1

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