I’ve got a 67 GT. It’s been running great all year but it has now started to get a little “miss” when cruising around 45 mph and with little throttle. This is very intermittent meaning it is not all the time and I can not reproduce it on demand. It accelerates great. I have also noticed a little miss at idle. I have already done the Petronix conversion of the distributor. The carb is a 750 Holley vac secondary man choke. I’ve been also playing with the idea of getting a 600 carb.
I was thinking of getting an aftermarket distributor for it but there aren’t many choices for an FE. MSD has one but you have to run a controller box and it has no vacuum advance and cost when done is probably around $800.
I had similar symptoms with my Windsor. Look for vacuum leaks. In my case it was intake manifold bolts.
I put on the torque wrench. I was surprised how many were under torqued. Very easy and the improvement was immediate. After driving the car several times the symptoms returned. I torqued them again and found 3 that were loose again. This time I added a little Loctite. Symptoms gone for a month now.
I was also wondering if I have a vacuum leak (With vac headlights and swing away wheel there are plenty of opportunities for a leak). I will check my manifold and I am also switching to electric motor headlights.
IMO you should see a difference when you convert to electric headlight doors. With 50 year old rubber lines and lots of connections plenty of opportunity for vacuum leaks. Do things one at a time so you see what makes a difference. Doing the headlight doors pretty high on the list. I’d put changing the distributor on the back burner and work through other items first. My guess is your distributor is fine.
I just checked my intake. Every bolt needed some tightening to get to 35 lbs. I also disconnected the headlight and swing away vacuum line. I will see how it runs tomorrow on the way to work.
If it has an intermittent miss, I would doubt it’s the distributor itself to the point it needs replacing
I would however look very close at every plug, then check every wire with an Ohmmeter (should be the same resistance per inch, not all the same), then look very close at the cap and rotor from the bottom side, as well as all your spark plug wire ends.
Next, if I didn’t see anything, I’d see if it does it with the vacuum advance disconnected and hose plugged. If it doesn’t, then look for a worn pigtail to the points, or in your case, the Pertronix. The vacuum advance moves that plate and sometimes wires can rub and wear, same with the ground strap.
The last thing would be the Pertronix itself. They are pretty good, but like all electronics, they can start to act funky as they get old.
So I went to work and back home. 30 mile round trip. I had no issues. Also, the idle sitting in the driveway didn’t have the miss either. So hopefully I found the issue being a vacuum problem.
One question for you all, Why would a vacuum leak cause a miss? Wouldn’t it just cause the engine to run lean? If is is a vacuum leak that made the car run poorly, I probably should tune the carb again after fixing it?