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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Triumph T120 Ignition Trouble


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1966 Triumph T120 Ignition Trouble 

Q: I have a 1966 Triumph T120 that’s been giving me ignition trouble. I’m running a Boyer, and find that the right plug fires just fine, but the left plug gets gas-fouled and doesn’t seem to be firing consistently. How can I troubleshoot this problem? - David Ragland/via email 

A: Your Boyer system fires both plugs every time the ignition is triggered. One cylinder will be on the compression stroke and the other will be on the exhaust stroke. The spark on the compression stroke will ignite the gas/air mixture and the spark on the exhaust stroke will do nothing. These are commonly called a “wasted spark” ignition. Since both cylinders are triggered simultaneously, and one of them is working well, I suspect a bad coil. The first test would be to switch the spark plug leads: put the right coil on the left and vice versa. If the problem moves to the other cylinder, you have a bad coil. The second test is to pull both plugs. Keep them plugged into the high-tension leads and ensure they are both grounded. Disconnect the leads from the trigger coils. With the power on, momentarily touch together the leads from the ignition box that go to the trigger coils. See if you get a strong spark on both plugs. If you get a strong spark that way, the problem is in the trigger assembly. The wires that run to the trigger assembly are subject to extreme heat and vibration, and it’s not unusual for them to fracture and cause trouble. Sometimes you can grab the trigger wire and wiggle it while the Triumph T120 is running and cause the Triumph T120 to falter. Other times the magnets on the rotor may have lost strength because of age or too much heat. Having a weak magnetic field will cause trouble. If you’ve gone this far you may as well pull the trigger assembly and test the coils and the magnets. You can test the trigger coils with a multimeter set to read resistance. The trigger coils should each show in the range of 65 ohms; any less and you have a short, much more and you have an open circuit. The magnets in the rotor should be able to support the rotor’s weight when hung from a piece of good steel, like a wrench. If one or both of the magnets are not capable of supporting the weight of the rotor, they are weak and should be replaced. MC