Backfire on decellaration.


Turn you fuel screw out (found on bottom of float bowl) Turn it out 1/4 at a time until the problem goes away. If you get out to 2.75-3.0 turns, you should go up one on the pilot jet.

The black on your plug is probably coming from another throttle position.



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, de-octopussed (Thanks Clark) EKP #4, 50PJ, 175MJ, 100PAJ and 65 SJ at 500-1000' Thanks James Dean!

The above is exactly correct. Try the fuel screw first then up the pilot jet one size at a time. To answer your other questions, lean condition means that there isn't enough fuel in the mixture, rich means there is more fuel than needed in the mixture. The deceleration backfire is due to a lean condition at idle throttle position. You could still be rich at mid to wide open throttle, thus your plug condition. What altitude are you at? If you're above sea level by >3000' it would be worth looking into a leaner main jet. Your owners manual has lots of info in the back about the different jets and how they affect your bike.

Thanks, I knew you guys would know what to do. I'll try the above mentioned corrections and see what happens.


2000 YZ426F

2001 CR500R

Also, a leaking exhaust header flange can cause the popping or backfiring.

My bike backfires on decellaration. A little background, bike is totally stock except for Hinson cluth and flywheel weight. Seems to accelarate fine, idle fine and has gobs of power. My local shop said that my bike is running lean, which jet affects when you let off of the throttle? Also when I looked at my plug it was dark and kind of wet looking, wouldn't that be rich. One last question, when you say lean does that mean not enough fuel or air?


2000 YZ426F

2001 CR500R

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