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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


1972 Triumph T100R Stalling Problem

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, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Stalling problem

Q: I have a restored 1972 Triumph T100R that stalls at idle. My mechanic added a Boyer electronic ignition, but left the 12-volt coils in. It has original electrics other than that, including the alternator/stator. He tuned the new Amal carbs twice and adjusted the timing twice, to no avail. Finally, he said I just have to blip the throttle when coming to stops to prevent the stall. I am wondering if this is an electrical issue or a carburetor issue. The carbs are brand new from England. They have been synchronized carefully. Could the problem be the 12-volt coils? I have heard that the Boyer prefers 6-volt coils. It starts right up every time and idles. It’s just when I run the bike normally then come to a normal stop it dies, as if it runs out of fuel or the spark to the plugs dies. This has ruined an expensive and long-term restoration. The mechanic said he has done everything he can and that it’s my riding style. I don’t believe that, as I’ve had Triumphs all my life and none have done this. Bill/Rhode Island

A: You definitely need two 6-volt coils wired in series for the Boyer ignition to work properly. Since it fires both coils at the same time (known as wasted spark ignition as one spark occurs in a cylinder that’s on its exhaust stroke) there is a need for the two coils to present the same resistive load as one 12-volt coil. Your current setup is too resistive and I think it is keeping the ignition from firing consistently at low rpm. If you’re reluctant to take it back to the mechanic, you should be able to replace those coils yourself. Just make note of the way they are currently wired and get two 6-volt coils of the same physical size to put in their place. MC

ottoknut
6/16/2016 10:35:16 AM

I would have thought that if high resistance in the ignition primary side was causing a poor spark condition then the condition would be more evident at higher RPMs/higher load, rather than at idle. It really sounds like a fueling problem to me; either too low an idle speed, incorrect idle air/fuel mixture, or an incorrect float bowl adjustment. However, having said that, I would definitely replace the ignition coils as a first step, as they are incorrect.