Technical Q and A for classic motorcycle maintenance and repair.
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.
Triumph/Lucas ET ignition
Q: While most people have given up on the energy transfer (ET) system, those of us doing a restoration for AMCA judging must live with it, as the bike must be started and the system must be within specs. As I started with just a frame and cases, the statement to “reassemble as removed” will not apply. I believe the rotor location pin on the engine sprocket is the key to its placement and as I was advised to put it at TDC with the pin at 8 o’clock. I have done so, but I still question why. The factory service books do not advise this, as best I can tell. Tom Gunn’s notes from TRI-COR service school do not address it either. Can you advise me further? — Dave Goldman/via email
A: The energy transfer ignition is an odd duck. It basically fires the coils on the upstroke, for lack of a better term, where a standard battery/coil system fires on the downstroke, so to speak.
I’ll begin by explaining how a battery/coil system works, then explain how the ET system works. The battery/coil system charges the primary circuit of the coil from the alternator and then holds the primary voltage until the points open (dwell time). Once the points open, the magnetic field produced by the primary coil collapses and induces voltage in the secondary coil. This higher voltage is what jumps the spark gap in the engine and ignites the fuel/air mixture. This system has a rectifier to change the sine wave AC alternator output to DC and a battery to level out the voltage.
The ET system keeps the primary side grounded until the points open, and then the voltage flows through the primary coil, exciting the secondary coil to produce a spark. This system has no rectifier to change AC to DC and no battery to level out the voltage, so to produce the best spark, you must time the voltage output of the alternator (sine wave) to be highest when the points open. Those old ET alternators have two sets of coils, one for lighting (ha!) and one for ignition. The rotor is placed as it is in relation to TDC so that the sine wave output will be highest when the points are to be opened. This is also why the advance unit is restricted on an ET bike compared to a battery/coil bike. Outside of the peak alternator voltage, there isn’t enough energy output to fire the coils. MC