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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


1978 Triumph Bonneville Has No Idle

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.

1978 Triumph Bonneville Has No Idle

Q: I am the original owner of a stock 1978 Triumph Bonneville that was working great until last summer when it lost its idle. This has happened before, and the problem was clogged idle jets. I took the bowls off and one was clogged. I cleaned and replaced everything. This made the idle better, but not perfect. I adjusted things and got it to idle right, but after a few minutes, it would slowly die as if I was running out of fuel in the carbs. I figured I may still have some residue inside, so I put some Star Tron fuel treatment in it. It started after two minutes. While I was waiting for it to warm up, it lost power and was running, but flat, reminding me of when I lost one coil’s power. Both cylinders were firing, and then it died. I have not been able to start it, and it seems to have no spark. A fuel smell is present and it was dripping fuel out past the rubber boots a bit after I wanted to make sure it was getting gas. Is there an electrical item that you would suggest checking? I still have points and the original Zener diode and rectifier. It has shown no signs of firing up since. It has 28,000 miles on it. — Bob Conrad/via email

A: Several possibilities spring to mind. Let’s work our way through from easier to harder. Since you say there seems to be no spark let’s start with the electrics. One of the easiest and least expensive tests is to just put in two new spark plugs. If that changes nothing then you’ll have to move on to more time-consuming tests. I hope you have a multimeter, as they make electrical troubleshooting much easier. How’s your battery? If it’s more than a couple of years old it is probably time for a new one. If it has a good charge then move a little further down the circuit. Is the fuse good? Sometimes those old barrel fuses look fine but the end cap has become unsoldered from the fusible link. Once you get past the fuse it is time to get out the multimeter and test for 12 volts. Are there 12 volts at the ignition switch? If so, turn the switch on and continue to the coils. Are there 12 volts at at least one of the coils? Running points, one set could be open and that coil would have no voltage. If you find 12 volts at the coils, push the kill button a few times and make sure it always comes back to 12 volts. Those Lucas switches get corroded after all these years and can keep you from starting or running. The next thing to do would be to pull the points cover and check for voltage at the points. You can turn the key on, then use a small screwdriver to open the set of points that are closed. You should see a small spark. If you have the spark plug for that cylinder out and the base of the plug grounded, it should spark at the same time. If you get no spark at the points go back to the last point where you had 12 volts and examine the wiring between there and the points, looking closely for loose junctions. That’s a good idea for this whole exercise; any time in these tests that you lose 12 volts, go over the wires from that point back to the last good 12 volts point, checking for loose connections. MC