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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


1967 Triumph 650 with Electrical Problem

 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, orsend an e-mail  with “Keith’s Garage” as your subject.   

1967 Triumph 650 with Electrical Problem  

Q:I’m hoping you can help me with an electrical problem I’m having with my 1967 Triumph 650. When I’m riding with my lights on, I keep burning out lamps. First my taillight goes, then, sometime later, my headlight main beam. The last thing to go is the pilot bulb, and then I’m riding in the dark. I’m running a magneto for the ignition, so my alternator output goes directly to the Lucas plate rectifier and Zener diode. I don’t have a battery since I don’t need one to start with the magneto. What is going on? — Eric/via email 

A: Sounds to me like your Zener diode has failed in the open position so that there is no voltage regulation going on at all. As your engine RPM rises when you ride, the voltage output of the alternator rises, too. In your case, however, its output isn’t being regulated to a maximum of 14 volts or so, and after a few hours of a 16-volt-plus feed from the alternator your bulb filaments weaken and burn out. Zener diodes are getting expensive and those old Lucas plate rectifiers get twisted and bent up, so this may be a good time to upgrade to a modern combination rectifier/regulator. Tympanium has made one for years, as has Boyer, and I’m sure there are others. They all install about the same, and the Boyer lets you start the bike without a battery if you are running a stock coil and points setup. With your magneto ignition, you don’t need that function, so the Tympanium may be a better choice. Check with one of our advertisers who sells British parts; they should be able to set you up. MC