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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Triumph Bonneville Low Voltage

Motorcycle Classics tech expert 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, or send an e-mail with “Keith’s Garage” as the subject.

Low voltage

Q: My bike is a 1977 Triumph Bonneville 750 Silver Jubilee. It is mostly stock, but I have upgraded the electrics with electronic ignition, a solid-state regulator and 6-volt coils. It runs great with the lights off or with the lights on as long as the engine is above 1,500rpm. My problem is that riding around town with the lights on, the voltage drops and won’t keep the battery charged. I haven’t tried putting the original coils in to see if the 6-volt units pull too much current, but they shouldn’t really be that power hungry. One old-timer I spoke to said the magnets in the stator could be weak after 35 years, but I don’t have another to compare it against. Do you have any ideas? What a great service you are providing — thank you! — Bart/via email

A: Well the old-timer is probably right. The rotor loses magnetism over time and so it produces less output from the alternator. The common test is to see if the rotor will hang off of a screwdriver by itself; if it does, it is deemed to be strong enough to run the alternator. A test of alternator output is also a good idea. Ideally, the voltage at the battery when the bike is running at or above 3,000rpm should be 13 to 14 volts. Any less and you will never charge the battery, as you have found. I’m assuming your solid-state regulator is a rectifier/regulator, replacing the Lucas plate rectifier and Zener diode regulator. Is your headlight the stock incandescent bulb, or is it updated to a halogen bulb that draws more juice? Short runs with the lights on is a strain on these old systems even if everything is in good order. You may find that you’ll need to use a trickle charger occasionally.