The purpose of this post is to get some input on where I should start looking first for the solution to the backfiring issue.
When my bike died, it was running beautifully.
Just rebuilt my first top end on an 08 yz450. I was meticulous in following the steps via the manual. Compression feels good, but is backfiring frequently. I blew the top end up so I remember how much backfiring was happening on decel. Backfiring is now happening on accel and decel. After a bunch of online research I understand that this can be from the following things:
Valves clearances out of spec
Here's some info about the bike. Valve broke, and got wedged in the valve seat in the cylinder head. Brought that head to a very good local machinist who cut the valve head out and repaired the damage on the cylinder head. He adjusted the valves for me. (By the way replaced the timing chain, thinking it was the chain that was the culprit leading to the damaged engine).
I have an aftermarket FMF header and silencer. The connection from the header to the mid-pipe has no sort of clamp or gasket to seal air in that area. I don't remember there ever being one. But if there is an air leak issue with the exhaust I could imagine that's where it would be. Is there supposed to be a mechanism to solidly connect those? I have not yet put gasket sealer back onto where I tried to take the silencer off to repack.
When my bike died it was back in September and was around 70 degrees. Starting the bike and hearing the backfire it was around 38 degrees.
My fuel screw is turned out 2.5 turns.
I have heard mixed reviews of people saying backfiring is caused by a rich mixture, and others saying a lean mix. Will someone give a solid answer to that?
Any help on this is appreciated. I've spent a bunch of hours working on the bike, which was fun. But really I just want to ride now. Getting help with ideas on what is most likely the culprit would be very appreciated.
Edited by desertwheelspin, January 08, 2016 - 06:01 PM.