Okay, so this is one of the more strange things I've noticed on my car: On a cold start, the car will sputter quite a bit during warm-up unless I sit there and flutter the throttle to coax it to stay alive. However, if I just switch on the parking lights (or headlights, or fog lights), the engine smooths out instantly and purrs like a kitten from dead-cold all the way through warm-up. What gives?? Once the car is warm, I can turn off the lights and it idles fine.

The car is positive-ground, with a brand new positive-ground Pertronix ignition and coil. The only thing I can think of (and this is a stretch) is that perhaps the voltage is low and disturbing the ignition, and when I turn the lights on, it's somehow activating the dynamo. I haven't had a chance to hook up a voltmeter to the car when it's parked so that I can test it during cold start.

Any other thoughts on how the lighting circuit could affect the cold idle?

Thx,
Doug

 

Well, not fully diagnosed yet, but I believe that the ignition circuit to the coil may be compromised, and turning the lights on may be allowing current to flow through another circuit to assist. I measured the current at the coil at cold rough idle, and when I connected a jumper wire directly to the battery, the voltage increased by 1/2 a volt, and the idle smoothed out. Pertronix says the ignition system should behave well all the way down to 8 volts and I was over 12, but there's no question that running a direct line from the battery to the coil materially changed how the engine was running.

More diagnosis to come as I trace down the various connections between the battery, ignition switch, fuse, etc.

 

What an interesting problem! I look forward to the solution even though I have nothing helpful to offer.