Motorcycle Repair: 1991 GSXR rough idle and fouled plugs, fuel delivery system, valve stem seals

Question
Hi Bill,

I have a 1991 GSXR 750 bought last year.
I'm having some trouble with a low and rough idle. I also fouled
a new set of plugs after about a month only.
Any tips on what may be causing my troubles? Do you see much
value in replacing my spark plug wires as a precaution?
Please advise.

I look forward to your reply.

Sincerely,

David Sanchez

Answer
Modern sport bikes are really not my forte' but based on your brief description, it sounds like you could have a fuel contamination problem. If the bike has been sitting for a long time, with old gas in the system, then the whole fuel delivery system needs checking. Replace the fuel filter, too.

Try to drain all the carburetor float bowls and see what comes out by catching the fuel in a small container. If there is water/dirt or old varnish, then the carbs need a good cleaning.

If the carbs are clean, the problem could be with the spark module controls, perhaps it is dropping two of the four cylinders under power or there is no spark advance, which is all electronically controlled. I certainly would check the ignition system... coils, wires and all connectors related to the ignition. Be sure that the battery is fully charged.

Yamaha makes some carb cleaner that you can funnel down the carb rack, let sit and then drain again. Follow the directions.

Other problems could be that the bike was rejetted incorrectly or there is oil consumption coming from valve stem seals or worn rings that are causing the fouling, as well. If you are having oil consumption problems, have a compression check or leakdown test done to determine the condition of your engine.

Bill Silver