Motorcycle Repair: backfiring intermittently, yamaha yfm 350, air fuel mixture

Question
Have a 1987 yamaha yfm 350 eru 4 wheeler.  I have set valves.  Timed it and it still backfires and breaks up.  Can you please  
help and give me some things to try and do.  It used to do this sporadically, now it is all the time.   Thanks Robby,  hazel9397@hotmail.com

Answer
Hi Robby,

Cylinder compression must be within service limits for the engine to run and carburetor to function properly. Same with valve clearances.

Check the appearance of the spark plugs. Compare your plugs with plugs images on NGK's website:

www.ngk.com/sparkplug411.asp

I suspect the air-fuel mixture is too lean and causing the performance problems. It's difficult to accurately diagnose the problem from the information provided.

Visit "www.atvfrontier.com" and post your question on the forums.

Respectfully,
Mark Shively