I have a 2000 honda 450foreman and when your going 60% or upwards of throttel for any lenght of time then knock of the gas she back fires esp if you do it going down hill.I HAD A 300FORTRAX and it use do the same when low in gas but this doesit even with a half tank the plug was changed about 200hrs ago would that be the problem? any answers would be greatly app.

 

Could possibly be running lean. Does your muffler rattle like the baffle is loose inside? Had one today where that was the case. I hit a bump w/ it and the baffle must of shifted just right and it had absolutely no power, let off the throttle and it would go again ok. I have replaced mufflers in the past w/ loose baffles and the machine would stop backfiring after replacing. Your muffler may be fine, but this is something I have seen before.

 

Sounds like lean idle circuit to me, too. Colder temperatures lean out the jetting, and yours might just be on the edge of lean.
Check for exhaust leaks and intake leaks, either one can cause the popping.
If the intake and exhaust look good, then adjusting the mixture screw richer (out), maybe 1/8 to 1/4 turn should fix it.
Do you use pure gas, or ethenol blended stuff? If you use the blended stuff, try a full tank of pure gas, and see if it gets better. The ethenol blended stuff is about 10% leaner than pure gas.
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