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.
Stalling 1981 Yamaha XS650
Q: I’ve been enjoying your magazine for a number of years now, but this is the first time I’ve needed to write for mechanical advice. I have an essentially stock 1981 Yamaha XS650 Special. Outside of some initial carburetor issues when I first bought it, the bike has performed flawlessly, until now. I try to put a few miles on it every week or so, and last week when I took it for a spin the engine inexplicably died about 7 miles into the ride. No stuttering, sputtering or misfires; it just quit as if I had inadvertently hit the kill switch. I tried to restart it, and although it turned over with both the electric and kickstarters, it wouldn’t catch. I checked the fuel lines, vented the gas tank and checked the plug wires, but that is about the extent of my roadside mechanical acumen. About 15 minutes later, as I was waiting for my distress call to be returned, I tried it again and it started right up. Hopping on I headed home, but it died again after about 4 miles. I waited another 15 minutes, it started up, and I made it home just as it died again, after about another 3 miles. I’ve taken it for a test drive around the neighborhood and the same thing happened again. Because the problem comes on so suddenly and affects both cylinders, I suspect the ignition coil (my understanding is that this bike has a single coil), but before I start randomly replacing parts, I thought I might seek a little guidance. Any insights into this problem would be greatly appreciated! — Jan Cordes/Yorktown, Virginia
A: I think you’re right about the ignition coil, and my guess is that it’s heating up and opening up either the primary or secondary side. It’s also possible the pickup coil in the alternator has gone bad. You can test the pickup coil by first disconnecting the plug between the air boxes. The plug should have black/white, white/red and white/green wires. With an ohmmeter, measure the resistance between the black/white and white/red leads, and again with the black/white and white/green leads. In both cases the meter should read between 550 and 850 ohms. Ideally you’d test this as quickly as possible after a failure, since your failures seem to be heat related. If that tests out OK you can proceed to test the ignition coil, but you’ll have to remove the tank to do that. Probing the primary side of the coil (the side not connected to the plugs) should result in a reading just shy of 3 ohms. Checking the secondary or spark side your readings should be somewhere in the 10,000 to 20,000 ohm range, depending on condition of the resistor plug caps. If after all this testing you find those components are fine, the only thing left is the TCI unit, which can’t be tested by ordinary means. MC