Technical Q and A for classic motorcycle maintenance and repair.
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Ignition or carburetion
Q: I recently finished rebuilding my 1978 Triumph T140E. I installed a Boyer Bransden Mark III ignition and set the static timing. I installed the crankshaft plunger tool to confirm that the rotor mark on the alternator is at 38 degrees before top dead center (BTDC). On my rotor, which is keyed to the shaft, there are two marks 180 degrees opposite each other. I first found the TDC mark on the flywheel, then, with the bike in high gear, backed the wheel up with my finger on the plunger until it clicked into the 38 degrees BTDC hole (full advance). This made the pointer on the primary drive cover line exactly up with one of the marks on the rotor, which I marked with white paint. I felt confident this was 38 degrees BTDC. I then lined up the magnetic reluctor on the camshaft taper with the hole in the Boyer plate for clockwise timing and tightened it. It seems to have seated and be tight on the camshaft taper.
I can start the bike, though it seems to take an extraordinary amount of effort; many kicks, an occasional kickback or a backfire through the carburetor. Once started, it seems to skip at times at idle, and when ridden seems to lack pep, though it sounds OK. When I applied a timing gun to it, I had to move the plate far counterclockwise (I am not sure if this means I am advancing or retarding the spark) to get it to line up at high rpms. Then, when I shut the bike off and tried to start it again, I got a wicked stronger kickback through the kickstarter and more backfiring through the carb. I put it back to the original (static timing) setup, and got the same result I mentioned first. Once the engine is warm, it starts up much easier, often in just a kick or two.
What could make it so the stroboscopically set timing yields results are so different from the static timing and cause so much trouble when starting? This bike is a late 1978 T140E with new Amal MK I premier carbs replacing the old Mk IIs, set with 106/200 jets. The battery, wiring harness, coils (two 6-volt in series), plugs and plug wires are all new. Thanks for your advice. — Aram N./Denver, Colorado
A: Let me see if I can help with your timing issues. First off, moving the plate counter-clockwise when the rotor moves clockwise means you are advancing the spark. Too much of that will definitely lead to kickback when starting. Make sure you have the correct color coded leads from the timing plate connected to the black box; reversing them will lead to timing problems. Make sure your battery is fully charged; a failing battery can lead to timing problems. If at all possible, power the timing light from a separate battery source. That will eliminate ignition noise in the circuit as a source of bad readings. If the timing plug and the alternator rotor line up, I’d say your static timing is set correctly.
You mention that the bike starts much easier when warm. This leads me to believe your problem may be more carburetor than ignition. The skip at idle and lack of pep also indicates that. Backfiring through the carburetor when starting is usually caused by a lean condition. Make sure the idle circuit is clean; new Premiers have been known to have clogged idle passages. New Premiers also come with stay-up floats: Make sure your float levels are correct. Does a simple press on the tickler allow you to flood the carbs to start a cold engine or do you have to press them repeatedly to make it work? A possible complication is the recent rebuild; are you sure the cam timing isn’t off? I hope this helps. Let us know how things work. MC