Motorcycle Repair: 83 490 it, air leaks, bill silver

Question
put new plug in when cranking over every 3or4 kicks it sounds like a shotgun whats up.

Answer
Kevin, I don't know much about a Yamaha 490, but when bikes backfire the usual causes are: that the cases are loaded up with fuel, there are restricted carb jets, spark timing is incorrect, the ignition coil is defective, spark plug cap defective, cracked spark plug insulator, ignition module or pulse generator are failing (if equipped), or there is an air leak in the intake tract.

Check compression, lay a fresh plug on the head and crank it over, looking for a constant and steady spark across the gap. If random/intermittent, then look for ignition-related issues. If steady spark, then check for fuel metering problems and air leaks. Make sure the whole fuel system is clean and has fresh fuel.

Bill Silver