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.
Q: I have a 1979 Triumph T140E with 17,000 miles on it. I’m the second owner and it has been well maintained. However, I’ve been stumped by an issue lately that I hope you can help me with. The bike starts very easily and idles well. When I’m accelerating, say from a stop sign, it has a stutter and occasional mild backfire at approximately 2,000rpm. It clears up at 2,500-3,000rpm and runs strongly after that to higher revs. If I roll on the throttle slowly, it does not do it. If I accelerate briskly, it will. It started doing this one day when I noticed it stuttered once. Then a few days later it did it a few more times. Then later it did it all the time, as it is now. I’ve adjusted the valves, checked all the electrical connections, cleaned out the carbs half a dozen times, all to no avail. The battery is in good condition. One day I put a new plug into the right side cylinder, and the problem went away. But 10 miles later, it came back. When I inspect the right spark plug, it is more black and sooty than the left plug. Could I have a bad or weak coil? Is there a way to measure/check the coil to see if that is the issue? Any other things I should be checking? What can I do to find and cure this situation?
– Steve Sullivan/via email
A: You can check the coil for primary and secondary resistance, but you may find it passes those tests only to keep acting up. That year had the Lucas Rita wasted spark electronic ignition, so the coils are run in series. Usually if one coil fails the whole ignition circuit quits working, although if one coil was shorting out instead of opening up, the other coil would continue to fire. Since it is wasted spark, and since you found that it improved with a new spark plug on the right side, it would be informative to swap spark leads from left to right coils and see if the problem moves to the other cylinder. The service manual for that year says to set everything by the left plug, and notes that the right plug will almost always be darker.