it sat for 2-3 weeks while i was rebuildin it, i ran it for about 15 minutes then it started this. however it was backfiring when i first started it so i reset timing a couple times and it had the timing right the first time. i pulled the float bowl and it had a little trash in it, i couldnt pull the carburetor apart the screws were stripping, im thinking about having the carburetor cleaned and jetted.