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Motorcycle Repair/1979 HAWK CM400T

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Question
QUESTION: I'M WORKING ON AN ENGINE PROBLEM. WHEN YOU START THE BIKE THE RIGHT EX PIPE GETS HOT TO THE TOUCH. THE LEFT GETS WARM ONLY.
IF I REVERSE THE PLUG WIRES THE RIGHT COOLS OFF AND THE LEFT GETS HOT. I PUT A DIFFERANT COIL ON IT AND IT DOES THE SAME THING. IF I PULL OFF ONE OR THE OTHER PLUG WIRE IT DIES OUT.WHEN YOU RIDE IT AND PUT THE THROTTLE TO IT THE CYL. WILL KICK IN AND RUN SEEMINGLY OK. AS SOON AS YOU BACK OFF IT RETURNS TO NO POWER MODE. CARBS ARE CLEAN. OR AT LEAST I CANNOT SEE ANYTHING IN THEM. BIKE SAT FOR 2 YEARS BEFORE I BOUGHT IT. RAN OK WHEN THEY PARKED IT.

ANSWER:
Hi Mark,

I recently had similar problems with another Honda
and this is what I did.

-Make sure fuel is not contaminated by a dirty fuel tank or moisture.
-Install an inline fuel filter.
-Check that the fuel flow from the fuel petcock valve
is sufficient to fill both carbs.
-If the idle circuits are plugged at all then it will not idle.
Soaking the carbs in Yamaha brand carb cleaner works
or some other safe for rubber brand carb cleaner.

Remove the idle jets and inspect closely as some have
small holes that need to be cleaned physically
with smooth wire like guitar string or replaced.

Synchronize the idle settings on both carbs, they should
be opening exactly the same at rest and as they open.
If one is opening first it may cause the other to falter.

Sometimes the air intake rubbers can crack and leak air/vacuum.
Check with carb spray around the intake.

Spark problems can be poor or dirty connections or weak battery.
Bad spark plug or plug caps can also cause problems.

Check all ignition connectors and clean them with
electronic spray like DeOxit or such.

If switching the coil wires changes the problem from side to side
then you may have weak ignition spark.
Check all connections including grounds and clean.

The coil is provided power from the ignition unit
so check and clean the connections there.

A timing light might provide clues as to spark
problems

The spark may be slightly hotter on one coil wire
and if the carb is running lean it fire when you
swap wires. This might still mean a lack of fuel
to the carbs or dirty idle circuits.

Plugged fuel cap vents can also restrict the fuel flow.

Use some new plugs if you haven't tried that yet.

Compression should be over 150 on both cylinders.

Wayne S
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---------- FOLLOW-UP ----------

QUESTION: I PULLED THE JETS OUT AND CLEANED THEM. DOES THE CARB BODY ALSO HAVE VEINS IN THEM? IF I PULL THE RIGHT HOTSIDE PLUG WIRE OFF THE BIKE KILLS. IF I LEAVE THE RIGHT PLUG OUT AND GROUND IT THE BIKE WILL START ON THE LEFT BANK. I CHECKED ALL CONNECTIONS TO THE CDI AND THEY LOOK CLEAN.COULD IT BE THE CDI? DO I DARE REVERSE THE TWO WIRES THAT FEED INTO THE COIL TO SEE IF THAT MAKES THE BIKE FIRE OR RUN HOTTER ON THE LEFT BANK? I MEAN RIGHT NOW IF I START IT AND LET IT IDLE SHE'LL BURN YOUR HAND ON THE RIGHT EXHAUST AND AT THE SAME TIME I CAN HOLD MY HAND ON THE LEFT PIPE RIGHT OUT OF THE HEAD JUST WARM. I ALSO PUT NEW PLUG WIRE ENDS ON IT. DIDN'T HELP.

Answer

Hi Mark,

If the bike will run on the left with the right plug
grounded then you must be getting some fuel
flow in the left carb which is good.

It seems that when you ground the plug it increases
the spark to the other cylinder.

Try varying the spark plug gaps and see if
it will run on both sides.

Coil should be grounded well and
primary ohms should be about 0.55-1 ohm
on CM400.

Motorcycle coils tend to be a bit weak
so I wouldn't be surprised if the coils
you tried are just weak.

I adapted a gm coil to a CT90 and it runs
like crazy and idles good now.

The important thing is to match the
original ohms so as to prevent damage to
the ignition box.

A weak CDI box could be possible too
but you would have to find one to borrow
and test it as they can be pricey.

The stator exciter coil charges the CDI
and then the pulse coil triggers the CDI
box to fire the coil.

Each component should be ohm checked
and then replaced if all else fails.

Electronics cannot always be tested and
sometimes replacement is the only
way out unfortunately.

You can vary the fuel intake by covering
the rear of the carb slightly to create a choking
effect. This might tell you if the fuel is lean
on one side.

I know it seems confusing but checking
each area carefully usually results in a solution
eventually. Very important not to pass
over something like goo in the fuel tank
as an example.  Some loose connector or
corrosion can weaken the ignition.
Take time to really examine things even
though riding would be more fun, good luck!

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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