Technical Q and A for classic motorcycle maintenance and repair.
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1978 Honda CX500 Ignition Problems
Q: I’m rebuilding a 1978 Honda CX500 and am having some ignition problems (the CDI ignition and stator are toast). I want to install an electronic ignition, but I’m worried about how much power it will take from the charging system. I have the stock headlamp and running lights. I was looking at the lowest-power system with enough spark for decent starting and running. I’ve got a Sparx ignition and Harley 3 ohm Sportster coil. The bike starts and runs, but do you think this is the best (lowest-power) solution, or is there something else? I haven’t got the rest of the bike together enough to do much testing on the current rig. - Greg Yotz/Gridley, Kan.
A: The Sparx EI shouldn’t draw much current from the charging system; it’s a low-power transistorized trigger for the coil. You don’t say for sure, but it sounds like the charging circuit of the stator is functioning but the ignition circuit(s) have failed. From 1978 to 1981, the Honda CX500 had a dual-purpose stator. There are three windings; one to charge the battery and the other two to provide the high voltage for the CDI. One of these windings is for low rpm and the other for high rpm. Over time, the heat generated by the engine causes the stators to fail. Usually only one circuit will fail; charging, low-speed or high-speed. If your battery won’t stay charged, the charging circuit has failed. If the bike will start but not rev past about 3,500rpm, the high-speed circuit has failed. If the bike won’t start, it is probably the low-speed winding that has burned up. It seems that in general, either the high-speed or the charging circuits give out. From what you say, your charging circuit is intact. If your charging circuit is still keeping up with the lights, I think you’ll be fine with the Sparx ignition. If sometime in the future you find the stator failing, you could replace the stator with one from ElectroSport and continue to use your ignition. MC