Intermittent miss at idle, likely cause?

[Robert M.]

I'll toss out this question for those of much experience.
I have an intermittent miss at idle, and probably at other rpm's I think. I believe it to be a drivers side miss, but my ears aren't as good as they once were.

The car is a 1970 350 L46, stock as the factory made it with the exception of the breaker-less single wire ignition, GM plug wires, and full time vac advance. It has the full complement of ignition shielding. The timing is set to 8 degrees at 750 rpm. Now pulling 15" at idle, with the proper vac advance can as Duke has recommended frequently. The plugs are AC R45.


Any starting points on finding an intermittent miss?

Electrical or fuel delivery?

Thank you,
Bob

[Erv M.]

Re: Intermittent miss at idle, likely cause?

Is the miss at idle or throughout the rpm range? It makes a difference.

[David H.]

Re: Intermittent miss at idle, likely cause?

If you remove the radio frequency interference shield on the distributor does the miss go away?

GM had a 1970 service bulletin on the rfi shield causing an intermittent miss.

[Duke W.]

Re: Intermittent miss at idle, likely cause?

Possible grouding in the primary or secondary wiring. As stated, check ignition shielding.

Also breaker plate ground wires are subject to opening up because they are constantly stressed by VAC 's movement of the breaker plate. They are soldered to the breaker plate and most probably have the original, which is way overdue for replacement.

You can test by aggressively moving the breaker plate while checking that its grounded. Also grab the ground wire with a dental pick and wiggle it around while checking ground continuity.

It could also be a damaged or leaky plug wire. Measure the wires resistances while wiggling them around - should be no more than about 5K ohms per foot.

Another test is to look at the engine while idling in a windowless pitch black garage. Look for blue flashes, which indicate a leaky wire that is dissipating energy to ground.


If the above checks don't reveal a problem, the breakerless ignition switch including its ground to the breaker plate is suspect. It's a "black box" that can act flaky. IMO one of the beauties of vintage cars is their simplicity including the single point ignition system.

One easy test is to replace it with a set of points and see if the problem goes away.

Other than whatever is causing the intermittent miss, sounds like you have a well tuned and optimized L-46.

Duke