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Motorcycle Repair/1979 HONDA HAWK CM400

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Question
WAYNE,I HAVE THE HAWK CM400 THAT ONLY RUNS ON ONE CYL. I TRIED A NEW CDI AND COIL AND HAVE THE SAME RESULTS. ONLY RUNS ON RIGHT CYL. REVERSE PLUG WIRES AND RUNS ON THE LEFT. I CANNOT FIND ANY BROKEN WIRES ON IT AT ALL. I TRIED TO TEST THE LEADS FOR OHMS BUT SEEMED TO BE OK. ANY OTHER IDEAS? WHEM YOU FIRST START IT COLD IT SEEMS TO WANT TO KICK IN AND REVS UP WHEN YOU REALLY PUT THE THROTTLE TO IT WHILE RIDING. HOWEVER WHEN YOU IDLE BACK DOWN SOMETHING KICKS OUT AND THEN FLOODS THE CYL. THEN ABSOLUTELY NO RESPONSE.

Answer
Hi Mark,

Sorry to hear your bike is still not cooperating with you.

If your bike wants to run when cold it could indicate
either a plugged idle circuit in the carb or Float level
off, it should just close the float needle when the float
bottom is 15.5 mm from the carb body.

The other possibility is a mechanical problem with
the valves being too tight or leaking which could cause it to run better
when cold.

Harbout freight has a leak down kit on sale for $39.95.
A cylinder leak down test can tell you if the engine
is leaking compression through a valve or head gasket.
It can be hard to find compression leaks without doing
this test.

If your carb is flooding the engine with fuel then you need a
new float needle and seat valve.

The way you describe it as kicking in when cold
sounds like either a lack of fuel or a valve problem.

When you open the throttle wide you are bypassing the
idle circuits needed for low speed running.
Also the engine may build up enough compression
at speed to start running.

I suggest trying to do a leak down test or
maybe you can temporarily swap the carbs
to see if it is a carb problem.

Sometimes carbs can be damaged beyond repair
and engines can also suffer mechanical problems.
The above tests might help in isolating the problem.

Hope something turns up.

WS
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Wayne S.

Expertise

Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.

Experience

Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

Publications
Canadian Motorcycling magazine article

Education/Credentials
Licensed Motorcycle Mechanic

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