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1970 Mark III Vapor Lock vs. Ignition Heat

Hi Bill,

I have had a 1970 Mark III for about 10 years. It is well used with 145,000 miles, but remains a great driver - has always been reliable.

I installed a re-manufactured Autolite 4300 (original equipment) carb on it and also upgraded to the Pertronix electronic ignition system and have also installed a rebuilt original alternator. The battery is about 3 years old, but spends winters on a trickle-charger.

She got a complete tune-up about two years (<1,000 miles) ago - new plugs, wires, cap, rotor, etc. To run at factory timing settings I run 50% racing fuel (110 octane leaded) plus 50% 93-octane unleaded. The above combinations have led to a car that starts, runs and drives smoother than any other Mark III I've ever driven (which is not a statistical sampling - I've driven only about four of them).

Last weekend, while stuck in stop-n-go traffic on the interstate, she died on me. I let it sit for 15-20 minutes and she started right up and got me the rest of the way home incident-free.

I wrote this off to vapor lock.

Yesterday, it did it again, three separate times - each time requiring a doubling of the cool-down time before she would re-start (1/2 hour, then 1 hour, then 2 hours).

Each time, I did a bit more diagnosis to determine it may not have been a fuel delivery issue (vapor lock), but instead may have been an overheating ignition system - as the coil, distributor cap, rotor and vacuum advance were all too hot to touch the last two times.

The last time, I also checked my distributor cap and rotor and even though they were relatively new, they were scorched at the contacts pretty badly - so I replaced them. Even after this, I had to wait another hour for more cool-down time before she would start.

In my 10 years of ownership, I've had a few incidents of worn-out wires needing replacement and I suspect this to be another example.

However, my Pertronix unit is also about five years (not more than 5,000 miles) old. I'm wondering if it might be dying as well.

Any other ideas for what might be causing excessive resistance in the ignition system? ...and do you agree this is where I should focus (not the fuel system).

(I'm ruling out vapor lock because after 15-20 minutes of cool down, I can visually see fuel flowing through the carb jets when I pump it - but was careful not to flood it).

Any insights you can offer would be appreciated.

Best regards....



Hi Chris -

We don't generally have any reports of vapor lock problems with Mark III's or any 460 cid Ford engines. I also have no idea if the fuel combination that you are using would contribute to vapor lock. The cars that we service use good local fuel that is easily available. As an experiment you could try a fill up with some local high octane only of a different brand as a test. I assume that your cooling system is in excellent working order and that the engine is not overheating when the engine suddenly stops. Of course as you suggest the ignition system could also be the culprit and the Pertonix being electronic could easily be a suspect. You will first need to determine if it stalls because of "lack of fuel" or" lack of ignition" during the actual stalling event and then diagnose that problem. If a fuel delivery test procedure for instance turns up poor fuel delivery when hot, the problem may be a failing fuel pump. Lack of ignition when hot may prove to be the Pertronix. The best test on the Pertronix is to substitute a good known unit. If I was to install a Pertronix in any of my vehicles I would always carry a spare unit in the trunk. You could also contact Pertronix and ask them how to test their module.

Sincerely -