Technical Q and A for classic motorcycle maintenance and repair.
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, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
A heat-related problem
Q: My 1980 Yamaha XS850 SG runs well until it warms up, but then it starts to miss, uses more fuel, and eventually stops running. Once it cools down it will do the same again. Back in 1980, I had one of these that did the same thing, and an adviser told me that the wiring harness had some added feature to suppress radio “noise” to improve radio reception. I think that he replaced the harness and everything was OK. Is this memory possibly correct or am I losing it?! — Robert O’Regan/via email
A: A few things are possible. Generally the ignition wiring either uses suppressor wire or suppressor caps. You can tell by pulling the cap off the coil and looking at the crimped-on connector at the end. Visible wire means the spark plug cap will be the noise suppressor, while a flexible ribbon or carbon string indicates suppressor wire. Sometimes with suppressor caps, the resistor contained in the caps goes bad, but this is rare. Suppressor wires can get brittle and the inner core string breaks down. Either of these things are simple and inexpensive to replace, so that would be a good starting point. A heat-related problem that affects all the cylinders also points to a central system problem like the igniter box. Those usually are only testable with factory tools, and often tested by swapping out a known good unit. If that doesn’t help, you might consider checking the fuel/air mixture. Put a new set of plugs in and go for a spirited ride on a lightly traveled road. Take along a plug wrench. When the bike starts to miss, hit the kill switch and coast safely to the shoulder. Pull the plugs and inspect for a rich condition. I suspect rich because you say it’s worse as it warms up and the old rule of thumb is, “Worse cold, lean condition; worse hot, rich condition.” Another good check would be valve clearances. If they are set too tight, when the engine warms up it is possible the clearance disappears and the valves don’t close properly. MC