Motorcycle Repair/83 490 it

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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

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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