2002 yz426 backfiring

I'm new to the 4 stroke world. My 2002 yz426 is backfiring on deceleration and tends to idle funny. Sometimes it'll almost shut off and sometimes it'll idle higher than it should. New plug, carb was jetted according to the last owner. Any ideas?ImageUploadedByThumper Talk1462374697.562605.jpg

First, check the idle mixture adjustment: http://www.thumpertalk.com/topic/271735-how-to-confirm-your-pilot-circuit-setting/

 

If that doesn't clear things up, check for air leaks, including around the fuel trim screw (the screw should have an O-ring seal on it).  If that fails, check your valve clearance.  Tight intake valves will cause this.

 

Note that decel backfire can be caused by the exhaust system leaking air, and commonly is.  Fix that before you dink with the carb if the first three steps give you a decent idle.

My 426 backfires that loud my dogs shit themselves

My 400 did the same thing gray is pointing you down the right path I sealed up my exhaust and set my mixture at two turns instead of 1.5 my plug now comes out good and no more back fire

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