I have a 99 450 ES that backfire's going down a long grade.I'll be going down a long grade,and right near the bottom,KABOOM!!!Talk about wake up the neihbours.If I'm going down a logging road,and let off the gas for a few seconds,it will backfire then also.

Would buying a higher grade gas help?It has a new air filter in it.(a month old)

Any suggestions?

 

Classic symptoms of a lean idle circuit. You did oil that new filter didnt you?
Check for intake and exhaust leaks. If there are none, then try using non ethenol gasoline, just use pure gas, and if that doesnt work, go about 1/8 to 1/4 turn out (richer) on the mixture screw.
Higher or lower octane wont effect this issue.
For a quick fix, and to verify its running lean...next time youre coming down that hill,pull the choke on, and itll stop doing that.

 

I'll check out what you said tomorrow.

Thanks for the help.

 

I have a 1998 foreman that does the same thing.

 

I tried that and it did not backfire.
I'll adjust the screw tomorrow and see what that does.

Thanks.

 

A 1/8 to 1/4 turn out should do it.

 

If it ever stops raining long enough I'll adjust it.

Thanks a'gain.