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Motorcycle Repair/1979 HAWK CM400T

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Question
I HAVE A 1979 HAWK CM400T. RUNS POORLY ON THE LEFT CYL. WHEN IDLEING. I REVERSE THE PLUG WIRES AND THE OTHERSIDE DOES THE SAME. I PUT NEW ENDS ON THE PLUG WIRES AND DIDN'T HELP. I SWAPPED THE COIL AND IT DOES THE SAME. ONCE YOU GET ER UP TO ABOUT 20 MPH THE CLY. SEMS TO KISCK IN AND RUN OUT GOOD. BACKFIRES A LITTLE AT IDLE. EXHUAST PIE A LITTLE WARM ON LEFT BANK. RIGHT SIDE HOT TO THE TOUCH. ALSO IF YOU UNHOOK ONE PLUG WIRE OR ANOTHER IT WILL KILL. HOWEVER IF YOU GROUND THE RH PLUG OUTSIDE OF THE BLOCK IT WILL START ON THE LEFT BANK BUT NO POWER AT IDLE. ANY IDEAS? CARB SEEMS TO BE CLEAN

Answer
Mark, the engine is a 360 cranks design so both sides are firing every stroke, so unless you have a bad spark plug cap on the left side, the problem isn't electrical The coil fires both plugs each revolution, one side on compression/power stroke and the opposite side is called a "wasted spark" because it isn't contributing to the power output.

Always check valve clearances and do a compression check to ensure that the problem isn't mechanical, like a burnt valve or worn rings. Most likely the idle jet is plugged up on the left side so it isn't getting fuel at idle until the throttle is opened and the slide starts to lift which starts to meter fuel from the primary main jet circuit, followed by the secondary main jet circuit.

http://www.cmsnl.com/honda-cm400t-1979-usa_model1049/partslist/E++20.html#result

The other possibility is that the air cut valve diaphragm is torn. You can remove the cover on the left carb and check it without pulling the carburetor rack.  Item #11 in above link.

The carbs really should come off for a good cleaning, new o-ring/packing, diaphragm inspection, air cut valve checks, idle circuit cleaning, etc.

I worked on one of these bikes a year or so ago and it was a PIA to get the carbs out. You have to remove the rear fender to get the air box back far enough to get the carbs out and you really can't work on them when the are on the bike due to space constrictions.

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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