Backfire - Help boiling down where to start first

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: 



Exhaust leak 

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

The first place to look is at your exhaust system.  If the carb is perfect, an air leak at either end of the header will cause exhaust backfire on decel.


You should also be aware that a correctly jetted YZ450 with a well sealed exhaust will crackle and pop "some" on deceleration as a normal behavior.  

Fueling is probably not correct for the bike , AM pipes don't help unless you are also willing to invest the PITA it is to get a good tune .

I would see what people have posted for jet specs for your particular configuration and year just to get a start and go from there.

But first make sure exhaust is good and sealed up with no leaks and same goes for airbox .

How did you set the valve clearances once you got it back together ?

And back fires can be rich or lean but tend to be a lean condition. Especially on race motors where there is so much focus on scavenging effects rich jetting could cause a poor cycle on one rev which causes a lean condition on the next resulting in a backfire.

Wow had almost the same thing happen to my bike

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