anyone help on this would be helpful,, i have 2001 raptor, it was just piped and jetted, now after the bike gets to operating temp., it backfires on deaccellration, am i running too lean? what is the main cause of this?

 

Your idle mixture is too lean. If tuning them out a little over 3 turns from all the way in doesn't fix it you need bigger pilot jets.

 

Could be an intake or exhaust leak, also. But richening the pilot circuit is probably the fix. This is from the mikuni website.




"4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires."

 

thank you all for your help, now i have a direction to go with

 

yep, what they said

I have this problem when I go to the slightly higher elevations without adjusting my idle screws richer

 

thanks you all, any other input would be appreciated, you guys r great