2000 magnum 500 wants to backfire thru the muffler. The unit was really hard to start so did the usual things, new plug, clean the carb in an ultra sonic. Checked out the fuel pump and realized that it was not getting enough fuel so replaced the pump. Now the machine starts right up but will not idle, will rev up then wants to backfire at just above high idle. It did not do this before the carb was removed except once in a while but not as intense. I am assuming(which you should never do) that it has something to do with the carb but I don't know just what it could be. As a side note the compression is around 120 with the throttle closed and burns a fair bit of oil. Have to top up after every ride. I have heard that some of these units could have bad cams. any suggestions would be grateful.

 

make sure you have 1/8" slack at the thumb throttle and no more! If this doesn't help,then pull the cam cover and just look at the center cam lobe. It should have the same elliptical shape as the two intakes lobes on either end of the cam. Make sure the valve clearance is .006 intake and exhaust also.Backfiring and no power is the main sign of cam wear and your model falls right in the year models affected (1996-2000) Her's a pic of a good and worn cam courtesy of another member.http://forums.atvconnection.com/pola...d-cam-pic.html

 

Check this for the idle first. http://www.ehow.com/video_4468404_polaris-atv-thumb-throttle-maintenance.html

Check the cam but from what I heard is they seem to back fire through the carb when they start to go.
Can,t be a coincidence that its worst after the carb was removed. Might want to make sure the carb boots where put back on air tight.