QuestionThe bike starts, idles and runs fine, as long as it is not in gear. The problem starts at about 3500-4000 rpm. It feels like it hits a wall and just won't go faster. It will eventually but is easier to shift up and stay under 3500-4000 rpm. to reach highway speeds. I have already cleaned and synchronized the carburetors, but may have missed something. The front mixture screw was only out about 1/2 turn, with the rear out further. It seems to run best with both out 1 1/2 turns. Front cylinder runs very rich and rear about right.
Sometimes I think it is not advancing the timing and other times I think I missed something in the carbs. Help.
Just acquired the bike with no history. Stock exhaust and air filter. Thanks in advance.
AnswerHi Michael,
There are a number of things that
could cause problems so start
with the basics first.
If you had the carb jets out the main
jets are different sizes so you
can't mix them up.
The carb for cylinder in the front is #125
the rear is #122.5 main jet.
I would check if your fuel pump is running
and check the fuel filter.
The carbs have coasting enricheners
which have rubber diaphragms that
can develop pinholes. These can
affect the fuel mixtures.
The fuel valve or petcock can develop
holes in the vacuum lines or the petcock
can leak fuel into the vacuum lines
and engine. This will make a overly rich
fuel mixture on the cylinder attached to
the vacuum hoses.
Carb slides have rubber diaphragms that
can develop pinholes or cracks that
will not allow the slides to lift
high enough for high speeds.
You can use a timing light to check
the ignition advance.
The main jets control higher speed fuel mixtures.
They must be cleaned with a smooth wire
to make sure they are clean.
Above the main are emulsion tubes
which can also be hard to clean.
I recommend soaking carbs in Yamaha carb cleaner as
it is very safe for most carb components.
Try to tune each carb until the plug color
is correct:
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm
Don't worry too much about the mixture screws as
they only affect low speeds.
Set them where it runs the smoothest.
Often spark can weaken because of
corroded ignition connections.
Use electronic cleaner from a electronic supply
to clean ignition connector plugs.
If your compression is low that would also
be a problem, over 130 psi is okay.
From about half throttle and up the
needle emulsion jet and main jets are
the ones in effect. These are the
jets in the carb centers.
Float level settings must allow the float
bowls to fill as well. Check choke settings.
Check plug caps also.
It may yet be a carb problem or fuel supply problem.
That rich cylinder may be the one to check out.
Good luck!
Wayne S.
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