Hi friends. At a car show on Sunday there was an old school guy with a Sun tuner in the back of a van doing tune ups . He's a very knowledgeable chap. I thought I'd get my 312 tweaked. All was well but while the car was idling the spark plug readings on the screen were completely off the scale. Massive peaks for each plug completely off the top of the screen. Strangely it returned to normal at cruising rpm. This has confused me. Is the oscilloscope showing voltage? Why would they be too high at idle? Plugs are Autolite 45 gapped at 40thou , it is running pertronix igniter and a pertronix coil. I increased the plug gap according to pertronix recommendations. He suggests all the plugs are faulty. Confused.

I'd guess it's the modern electronic unit messing with an old scope that wasn't designed for such high voltages. If it's a petronix 3 with multi spark, who knows what the scope would interpret it as. I know my MSD units confuse some of my old timing lights.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

It's extremely unlikely that all 8 plugs are bad.

I agree, probably more modern electronic ignition not registering properly on old diagnostic tech.

If all 8 plugs appear to be bad, then it is probably something they all have in common which is actually the problem.


Lawrenceville, GA

Yeah, cheers guys. Car runs fine but he said it was a problem. Odd as my friend has exactly the same ignition as mine in his 59 Galaxie and it showed normal. I would have thought a good spark was beneficial. Thanks.