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 the subject.
Q: I recently bought a 1974 Triumph Trident T150V, which I have had no problems with up until a week ago. Returning home from a run it began missing at 3,000rpm through all the gears. Initially I thought it had sucked in moisture through the air cleaner when I got caught in a major downpour of rain. I limped home, cleaned and dried the air filter, and replaced the spark plugs for good measure, but this did not rectify the problem. It didn’t have any problems starting, but as soon as the engine hit 3,000rpm it once again began running rough. Then, without warning, the engine completely cut out and has not sounded like it even wants to fire since. I unhooked each lead, inserted a spare spark plug and cranked the bike over, but there is no spark at the plug across any of the cylinders. I presume I have a coil, Boyer ignition or battery problem but do not know how to test these components in order to isolate the source of the problem. I would be appreciative if you would be able to shed light on my current predicament. — Kere Cookson-Ua/via email
A: There are several ways to test all these components, so let’s start from the simple and move on to complex. Boyers can be sensitive to voltage, so make sure your battery is up to snuff. Turn the headlight on and make sure it stays bright for at least a minute. If you have a multimeter you can check the voltage. It should stay at about 12 volts for that time. Then make sure you are getting power to the Boyer ignition. The kill switch on the handlebars is known to get corroded and stop the bike from running. In addition, the ignition switch can get corroded, too. I’m assuming you still have a positive ground electrical system. Make sure you have a good ground, both from the battery to the frame and from the Boyer. Try running a wire directly from the negative side of the battery to the white lead on the Boyer. Check for spark as you did before. Any luck? You’ve already tried the cranking test with a spare spark plug, so we’ll move on to a test that bypasses the pickup coils. Disconnect the two wires that go to the stator plate on the timing side. Pull all three plugs on your Trident or pull the plug wires and use new plugs in the sockets. Ground all the plugs to the engine. Turn the key on, then touch the two leads from the Boyer box together. If you have a spark at all three plugs then the pickup plate is faulty. Sometimes, due to vibration, the wires will fracture inside their insulation on the plate. If you don’t get a spark, it could be that you have a bad coil. Since the coils are in series, one bad coil can keep the rest from working. Try bypassing all but one coil and repeat the previous test. Do that for each coil. Did you get a spark? If you did, one or more of your coils is bad. If you get no spark after trying all the coils, you probably have a bad Boyer ignition. The only way to test the black box is to substitute a known good one in it’s place. I’m betting you find the problem is in the ignition switch or the kill switch. MC