I am new to Thumper Talk. Im not super mechanically inclined so I would greatly appreicate any help. My bike backfires when letting off the throttle and is Hard to start when cold, and as of just recently wont Idle at 2000' but will at when around 6000'.

Im not exctually sure what is in there now. (Just bought the bikenot long ago)  

Model:07 YZ 450F

Main Jet:Stock

Pilot Jet:Stock

Starter Jet:Stock

Fuel screw/Setting:stock



Pipe/muffler:FMF Mega Bomb Header and FMF Powercore 4.1 muffler

Altitude Range:2000-6000

Temp Range:AZ 50-110

Humidity Range:very low


Is the pilot jet plugged-up?

Not sure I'll have to check

Excessive decel backfire (some is normal) is caused either by an air leak into the exhaust system (check that before rejetting to relieve this problem) or lean pilot jetting. 


1) your bike runs better at altitude than at 2000 feet


2) Engines run richer at altitude because of the lower air density


Ergo, your bike appears to be lean at idle, which condition is improved by moving to higher ground.


Start by cleaning up the pilot circuit, and consider going up one size on the pilot jet, just for starters.

Try 6sigma

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now