I usually work on automobiles and outdoor power equipment and ran across a 2003 Polaris Scrambler 90.
At first it did not start at all and ran a compression check and was 65 psi. I removed the piston and cylinder asy and was completey scored all around. The air filter was pluged badly with dirt. The fuel was bad. I cleaned the fuel tank, removed the carb. and dropped the bowl and cleaned out the jet and replaced the piston and cylinder asy. with a new spark plug.
Ran the unit and ran fine the only thing after acceleration it backfires upon decelleration. The only thing I could think of the moment is that the carb adjustments is off and running lean. Usually i know of a high speed adjustment and low speed adjustment but do not know exactly how the adjustments on this carb is set up. Need some help? Thanks

 

Normal air fuel screw setting is 1-1 3/8 turns out. Yes backfiring is usually a lean condition,can also have air leak from crank seals,or intake cracks,plus can backfire from excessive throttle slack(fuel charge builds up and ignites) Have had older 250 and 400 2-strokes backfire and blow off exhaust pipe from header when slack too much. Check these out first. Adjust carb and cable first ,may be the problem. Good luck