Motorcycle Repair: 1979 honda cm400t, poor vacuum, engine vacuum

Question
I have low power at the start of the throttle it lags at first but when the rpms go up the engine seems to catch up and then i have lots of power so taking off is somewhat slow what can cause this and how its it fixed

Answer
Hi Donald,

This problem can be related to a few things.
Often it is the slides that go into the
top of the carburetors getting sticky or
their gaskets or seals are leaking air.

The carb works on a vacuum principle so
the carb slides depend on engine vacuum
to lift. A worn engine can also
have poor vacuum.

If you check the pipe temperatures
they should be about the same otherwise
one cylinder may not be pulling it's
weight which can make it bog.

All the common tuning areas should be checked
also. Check the air filters, spark plugs, plug caps
and clean the carbs thoroughly.

the spark plug color can give you clues as
to what is happening inside the engine
cylinders.
Compare your spark plugs to this guide:
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

The bogging can be dirty carbs, slides leaking
air or sticking. Intake air leaks, one cylinder
cutting out at low speeds due to any of the above.

If the bike has been parked for more than
a month the carbs could be getting gummed
up inside as the fuel these days goes bad quickly.

Any moisture in the fuel can cause low speed
problems as it blocks the idle and needle jets.

If you have a mechanical ignition advance make
sure it is not stuck in the advance position.
It has springs that should pull it back
to low speed timing position.

Good luck!
Wayne S.
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