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, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject.
Yamaha XV920 Virago troubles
Q: I have a 1983 Yamaha XV920 Virago. Two weeks ago, I over-revved the engine, and it ran just fine for the next 10 miles. After 10 miles I had to stop at a stoplight and the engine stalled. When I went to start it, my starter wouldn’t engage. The starter had been going out like Virago starters do. I replaced the starter and it works great. Now the bike will start but will only run on the front cylinder. My manual said I should clean the carburetors, which I did with carburetor cleaner, and I blew everything out with compressed air. The starting circuit was dirty, but the rest was pretty clean. Now the bike starts and runs better, but still just on the front cylinder. What should I do? I don’t want to throw parts at it until I get it running. Any help would be great. I use this bike to save gas, plus I like it and miss riding it. — Bill Porter/via email
A: Since you mentioned over-revving the engine, let’s start with some simple engine checks. Too many times I’ve looked for complex solutions to simple problems, wasting a lot of time in the process. The first, simplest and least expensive test is to change your spark plugs. Sometimes a spark plug will show a spark in free air but refuse to fire under compression due to some minor fouling. A compression test on the cylinders could also help pinpoint the problem. Your engine in good shape should show compression of about 135psi per cylinder, and there shouldn’t be more than about a 20 percent difference between the cylinders. Poor compression can indicate a blown head gasket, piston ring problems or leaky valves. A leak down test after the compression test can further isolate low compression to any one of those causes. BikeMaster sells a universal compression tester kit, and you can get a leak down tester from Harbor Freight or Pit Posse. You may be able to borrow these tools from a local auto parts store. You also mentioned changing the starter, so check and make sure no wiring was disturbed when you installed the starter. After that, you can troubleshoot the rest of the ignition. You can usually test the coils with a multimeter as a first step to determine if they are bad. Set the meter to read ohms, disconnect the leads to the primary side of the coil, and connect the probes to the positive and negative terminals at the coil. Your meter should read approximately 2.7 ohms. Do the same thing testing the secondary side of the coil, connecting one probe to the spark plug lead and the other grounded to the coil’s negative terminal. Here the reading should be approximately 8,500 ohms. Less than that and the coil is shorted, more than that or infinity and the coil is open. Either way it won’t produce a spark. If your coil tests OK you’ll have to test the pickup coil and ignitor unit. The easiest location to test the circuit is at the ignitor box under your tank. The pickup coils should read between 124 and 186 ohms, and again, wide variance from this indicates shorted or open circuits. On your bike, the brown/green wire is the rear cylinder and the red/blue wire is the front cylinder. If possible, I’d swap the coils to see if the problem moves with the coil. Also, try moving the trigger wire and spark plug wire from the front coil to the rear coil and see if it still won’t fire the rear cylinder. If the bike then runs on the rear cylinder only you’ll know it’s the coil or spark plug wire. Keep at it and you’ll find the issue. MC