Motorcycle Repair: 1980 KE 175, crankshaft seal, fuel mixture

Question
I've had this bike for about 11 years now, it's been tuned up and carb work done to it about 3-4 years ago. For the past 5 or so years. I've noticed when running it for 30 mins or more, when I come to a stop and pull in the clutch. The bike revs up like the throttle is held wide open, and I have to kill the engine either with switch, or higher gear engaged and popping the clutch with brake held in. I don't have a tach since the previous owner took it off, yet, I've been riding for the past 17 years and to me sounds like if I don't kill it...it will blow something. I've traced the lines and actuated the throttle (while running and while turned off), nothing is hanging up. The idle is correct and there is no hesitation, it doesn't act as if it wants to engage in full throttle on its own while riding. It only does this when I come to a stop and engage the clutch. A side thought, a few random times I've noticed that it didn't feel as if it was going to high rev, when I slowed the bike by down shifting and then engaging the clutch. I haven't worked this theory yet, but from memory it didn't seem to recreate the problem by doing this procedure.  I'm just stumped. What are your thoughts on the issue?

Answer
Hello Al,

Whenever a bike wants to rev high and the
throttle is closed it indicates
a lean fuel mixture for one of a few
reasons.

One possibility is the crankshaft
seal under the flywheel starting
to fail. The engine can pull air
into the crankcase and it leans out the
fuel mixture going into the cylinder
and causes the high engine speed.
92049-1482 SEAL-OIL

The other thing that can cause lean mixtures
is a cracked carb intake rubber tube
or any leak where the carb attaches.
Check with carb cleaner or such.

Another possibility is just worn
carb slides and needle jets or
a bad reed valve on the intake.

Some fuel petcocks have a vacuum line
that can draw fuel when faulty.

Yet another reason is a leaking head or
cylinder base gasket sucking air.

Even a stuck ignition advance on the
backside of the flywheel can
cause high engine speeds at the wrong time.

If you check all the above you should find
the culprit.

If it was the old rotary valve engine
I would say check the fibre rotary valve
for scratches as well.

Good luck!
Wayne S.
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