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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


Triumph Bonneville T120 Stumble

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.

Triumph stumble

Q: I have a 1965 Triumph Bonneville T120 with 1,000 miles on it since it was rebuilt. This engine ran fine for approximately 600 miles, but then began to stumble and lose power after about six miles of riding. When I stop for a minute or two, it will start and run again, but only for a half mile, then it stumbles and loses power again. It will not generally get above 20mph once it begins to act up, but it does stay running. It will stall once at idle. It is running one Amal carburetor with a Pazon ignition. The battery is charging 12.7-13.2 volts (approx.). I replaced the coil. It ran strong for about 200 miles after that, but then started the symptoms all over again. The plugs are evenly colored with no carbon build up to speak of, nor any indication of running lean. Popular opinion is there is an overheating problem affecting ignition performance. It acts as if it is being starved of fuel, but there appears to be no reason to suspect the carburetor. I have good fuel flow from the tank. I even checked the cap to ensure that the venting wasn’t plugged. Thanks for any assistance you can offer. I’m stuck. — Jonathan P. Young/Fort Drum, New York

A: It could be a compression issue. Since the problem started soon after a rebuild, I have to ask if you’ve retorqued the head bolts. Depending on the type of head gasket used, you may have to do this a few times. You can check for low compression with a compression tester or a leak down tester. Another possibility is overheating of the electronic ignition trigger in the points cavity. You can quickly rule this out by pulling off the points cover and going back out for a ride. That will keep the trigger assembly cool. If the problem disappears you’ll know to replace the trigger assembly. Or it could be a problem with valve clearances; everything runs OK until the engine warms up and valve clearances start to close up. Once that happens your valves won’t seat as they should, leaking compression and causing the engine to lose power. Double check your cold valve clearances. They should be 0.002 inches for intake and 0.004 inches for exhaust. As a test, if they check out OK you could add 0.001-0.002 inches to each setting and see if that makes a difference. If it does, then you know there is a problem with the valves. If it’s a problem with the head gasket, you’ll probably have to pull the head and replace it if it’s composite, or reanneal it if it’s copper. Then be sure to do the retorquing to keep it sealed. Good luck. MC