Backfiring YZ426f

All i did was change the plastics over to '03 - '05 yz450f (tank, seat, airbox, boot, subframe, etc...)

I did put fresh from the pump 100 octane in the new tank (which was empty, but it did smell like old gas...)

I started it up (it did take quite a few kicks than it did 2 weeks ago), let it warm up, and it was backfiring like crazy - on the throttle, deceleration...

there was an exhaust leak from the head pipe/muffler junction...i did tighten up the clamp, the back firing did lessen, but it was still there...

Would the higher (? - i really don't know what was in the original tank) octane gas have that effect?

Would the air boot from the air box to the carb cause this? the one for the 450 is a little bit differently shaped...

Would the higher (? - i really don't know what was in the original tank) octane gas have that effect?

No, absolutely not.
Would the air boot from the air box to the carb cause this? the one for the 450 is a little bit differently shaped...
No.

Your problem is probably one or both of two things. Either the exhaust system is leaking at the head and/or the joint of the header and midpipe, or your pilot jet is partially clogged with fuel varnish.

Ok I am haveing the same problem... Other then the back firing the bike runs fine. It is mostly back firing when decelerating or down shifting. I have a full after market FMF exhaust with NO leaks. The other night was the first time I had ridden the bike at night and noticed not only that it is back firing a little but shooting a nice flame from the exhaust. I have taken the carb off and cleaned it several times. I changed the needle and seat as well. PLEASE help.

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