89 yz250 backfires only

[dmstafford79]

1989 yz250... Kick it and kick it and wont start.. Has fuel compression and spark. About every third kick it backfires and blows the pipe off the cylinder.. Help please? :bang:

 

[arnego2]

Timing or the pipe is closed, when did you clean the pipe?

 

[dales298]

Check the flywheel key. The last time my bike did that it had a broken key.