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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Triumph Bonneville Idle Jets


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 your subject.  

1978 Triumph Bonneville Idle Jets 

Q: I am the original owner of a stock brown and gold 1978 Triumph Bonneville that was running great until last summer. I lost my idle. This had happened previously because of clogged idle jets. I took the bowls off and one was clogged, so I cleaned and replaced everything. The idle was better, but not perfect. I adjusted and got the idle right, but after a few minutes it would slowly die, as if it was running out of fuel. I figured I may still have some residue inside, so I put some fuel treatment in it. It started, but after two minutes while waiting for it to warm up, it lost power. It was running, but flat. Both cylinders were firing, but then it died. I have not been able to start it since, and it seems to have no spark. Fuel is present. I still have points, the original Zener diode and the rectifier. It has shown no signs of firing up since. - 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. You’ll want a multimeter, as they make electrical troubleshooting much easier. How’s your battery? If it’s more than a couple of years old, it’s probably time for a new one. If it has a good charge, then move a little farther 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, use your multimeter to test for 12 volts. Is there 12 volts at the ignition switch? If so, turn the switch on and continue to the coils. Is there 12 volts at least at 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. Next, pull the points cover and check for voltage at the points. You can turn the key on and 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 volt point, checking for loose connections. MC