Just found this forum and thought I would throw this question out to the group. I bought a 1998 300 new and it backfires when letting off the gas after riding it hard. It's really bad when coming down hill. The dealer told me that all the 300's had this problem and not to worry about it. The extra noise is what bothers me. Has anyone else had this problem and can I do anything about it?

 

Hi,i have a 300 and it to used to backfire alot.I had jet changes,valve adjustement and still backfired.It's when i changed my air filter that i noticed it didn't backfire as much so now i don't put as much oil on the air filter and it don't backfire at all now.

 

I have 98 300 too. It backfired whenever i let off the gas. i changed my jet down one size and it stopped. I have been told that AC over jets the carb to help compensate for cold weather. A smaller jet was a couple of dollars so it isnt expensive and it is easy to change. Good luck

 

My AC 250 dose this too where can I get the jet, and I beleive that the charred white ashy look on the spark plug is related.

Am I right?

 

I used to have a 300, it did it too. My 400 I have now did it a little, but not as bad.
I bought a dynojet kit for my 400, because I've used them in a few other performance bikes, and the throttle response, performance, and idle have never been better, and no backfiring at all. I highly recommend it, for machines. Its $50, but worth it. The needles in the Cat carbs are their biggest downfall - the Dynojet needle is specifically designed for each model, for optimum fuel/air mixture thruoghout the rev. Instructions are spot-in - follow them exact for elevation recommendations.
On the cheaper side - the idel mixture screw can be tweeked a little to calm the popping down, and get a better idle. It is on the bottom, just in front of the bowl. Turn it 1/4 turn at a time, one way or the other, until you achieve the best idle with the least popping.

 

chspyder, charred white and ashey sounds like more of a "too lean" condition than a "too rich" condition. When an engine is charring white ash on the spark plug it is getting WAY too hot. It sounds like you need to go up a size in the Main Jet... That's where I would check first.
Rich conditions are generally indicated by a thick black soot on the spark plug.

EtherNut

 

thanks all got it fixxed learned a lot but mostly learned 2 things choke sticks if any moisture getys in it, and also that SOME atvs have two screens in the air box and one should be removed, check with your dealer to see if you qualify, if so remove one and allmost all of your problems will be fixxed!