Dirt Bike Modifications Forum

HOWDY. QUESTION.  CFR450R WILL NOT IDLE WHEN COLD. WHEN REVVING, LIKES TO BACK FIRE ON THE DOWN REV.  NO GRUNT DOWN LOW BUT COMES UP QUICK AT HIGH RPM'S.  LIKES TO QUIT ON TRACK WHEN OFF THE GAS, THEN HARD TO CRANK.  HAVE TO KEEP IDLE HIGH TO KEEP RUNNING ON TRACK. ANY ANSWERS? AND PLEASE BE SPECIFIC, CONSTRUCTION WORKER, NOT NORMALLY  A MECHANIC. THANK YOU KINDLY. 

initially i would point the finger at the jetting? (is it jetted, did you buy it new?) but it may also be a bad
air/fuel mixture. get back to me it'll help me to know some about the jetting, your elevation, carb work?, exhaust?  

I  dont really know what altitude your at but back firing and hard starting sound to me like possible valve isssue. If you have the basic tools, torque wrench and feeler gauge I would suggest doing a check of your clearence to be safe, and if its in speck you know that its the carb for sure, which by the way can usualy fixed with a cleaning or minor rejetting or adjustment. 

         

Yes, you have a jetting problem. Your bike is jetted to lean, backfiring on decell is a big indication. Another problem is actually having your idle adjusted higher, that just leans out the bike more. You should put the idle back to normal, and adjust your air screw. Screw that in a little bit, and bring your fuel screw out just 1/4 turn.

Oh yea, you should do this before you ride much more, or you could pop your piston a good one. And when you change the jetting, put in a new plug and you should be good.