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Motorcycle Repair/honda 90cc cycle carb.adjustment

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Question
My Honda 90cc cuts out badly at high r.p.ms.
Is this fuel ratio? How may I adjust?
Is there a fuel filter that may be plugged? If so, where is it?

Thanks for any help you might offer.  Rich

Answer
Rich,  Assuming that the engine has a good 150psi compression, high speed misfiring on the little 90s is often a matter of not having a fully charged-load tested battery in place in conjunction with an operating charging system. These are battery operated ignition systems.

You didn't tell me which variation of Honda 90 you have... S90, CL90, SL90, CT90, ST90 or even a C200/CT200 pushrod engine. The fuel system filters are generally part of the petcock/fuel valve on most models.

The other overlooked item is the vent hole in the gas cap. If the vent hole is blocked with old gasoline deposits, the fuel flow will be interrupted at high speeds. The tiny vent hole is just on the outside edge of the gas cap gasket.

Other possibilities are if you have altered either the intake system/air filter or the muffler/exhaust pipe. If these are not originals, the carburetor jetting is often out of range, requiring a larger main jet and sometimes a change in the needle clip position.

If the ignition timing is not set correctly or the spark advancer is jammed or sticking, this will affect high speed running as well. Point gap should be about .014" with the point plate positioned so the points just OPEN at the F mark on the rotor.

Bill Silver

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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