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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


Fouled Plugs: Suzuki T20

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Fouled plugs

Q: I have just restored a Suzuki T20. The bike will start on the first kick and it idles well, but the right exhaust sputters out oil and gasoline and the right spark plug is fouled, black and oily. The right cylinder makes a popping sound. I tried advancing and retarding the timing, but it didn’t make a difference. I think the problem is the coil for the right cylinder. It has new rebores, pistons and rings. The crankshaft was rebuilt with all new seals, bearings, etc. The battery has a full charge. Another symptom is the compression. With the kick it shows only about 50-60psi on both cylinders. Should it be higher? The specs say 130-135psi, not sure how it can go this high using the kickstart lever? Any ideas would be most appreciated. — Osvaldo Castagna/Australia

A: With a compression reading that low, I have to ask: Did you hold the throttle wide open? You could get a low reading like that if the throttle slides were closed. Holding the throttle open allows the engine to get the full amount of air needed for compression. Your compression must be higher than 50-60psi in reality if the engine is running at all. Try it again, kicking until the gauge indicator stops increasing. As for the right cylinder, make sure the carburetor float isn’t leaking. If it is, it will sink in the float chamber and cause that side to run rich, which would explain the fouled plug. Assuming it’s made of brass, there’s a simple test for a bad float: take it out of the carb and shake it. If you hear fluid sloshing around inside, it’s gone bad. If the right side has been overfed, so to speak, you’ll have to get the engine good and hot to burn out all the excess fuel and oil in the crankcase on that side. You may find your problem takes care of itself after a good long ride. Carry a couple of extra plugs and a wrench, just in case. MC