Bob I had the exact symptoms you described on my newly rebuilt engine. I am using the original ignition system, (all analog.)
In my test drives for about 20 min I had smooth acceleration great power until the car got to operating temperature at which point it would start missing at higher rpms. In about 10 more minutes of driving it would be missing so badly it would no longer accelerate. I would end up coasting to a stop with the engine stalled. Trying to immediately restart the car was not successful as there were no signs of ignition. After waiting about 20 minutes I attempted to restart the car, and it started right up. I was able to get the car home. The first time this happened I thought it might be a fuel problem or timing etc. I reset timing checked mixture etc. . The second test run same thing happened. I pulled a plug wire to watch for a spark while attempting a start. There was no spark to be found on any of the six cylinders. I put my hand on the coil which gave me a nearly second degree burn on my fingers. Well, that seemed atypical. I waited 20 minutes for the coil to cool down after which I got an immediate start. By the time I got back to my home (10 minutes later) the same symptoms presented. The car died in my driveway as I was pulling in. I got a small bag of ice and draped it over the coil. In about 5 min, I was able to start the car and it ran fine until the coil overheated and ceased to function again. I tried a new coil but got the same results. So not a bad coil but something is creating high resistance making the coil overheat leading to failure.
My Discovery: (self-inflicted most likely)
Nearly all of the acorn nuts on the distributor cap were a little more than a 1/2 turn from being seated leaving a gap between the ends of the plug wires and the distributor. This significantly increased the resistance in the circuit resulting in the coil overheating. So I didn’t trust my fingers this time to seat the acorns and used a small pliers to make sure they were fully seated.
Test drive - no trouble found - coil is touchable… Engine is running great and had it out for almost 45 minutes before i brought it home with no forced roadside stops. All good.
Looks like you have a lot of great ideas from the group. Hope it’s a simple fix for you.
I vote electrical issue. Try replacing condensor. If you are running modern carbon HT leads, they can fail. My old favorite is check the earth strap from engine to body.
This is an absolute identical description of my problem except that my engine didn’t stall (yet).
And now the real surprise: I also discovered that I could turn all the acorn nuts on the distributor cap by at least a 1/4 turn. I discovered this unfortunately only after I had completely removed the distributor from the engine (which is a nightmare in it self).
I’m now in the process replacing the capacitor and points as I noticed that the points didn’t look very nice for an engine that may have run for 3 hours only, and I didn’t replace the capacitor during the restoration!
So my problem might be even “multi causal” but I assume that your description is the main cause. Hope to have the car running in a few days and will report back whether you’ve solved my problem.
Bob, great and good luck! Will look forward to the final outcome. Clive also makes a great point on the earth strap. I experienced
that issue on an old 66 S type years ago.
since you have cap off…checking wire nuts…check the resistance of the plug wires…as I have discussed prior…you do NOT want the high resistance modern wiring…Resistance is measured per foot…so you want very little resistance. The gap described in nuts…is also like resistance…it all adds up…makes the coil hot.
Nick
Made a 45 minute drive without any problems. The engine behaves very well and has lots of power and the pick-up from low revs is perfect without any hesitation.
Thanks to the great advice from AV8R, it was clear that due to a gap between the brass “slit” washer in the 6 acorns and the fixed contacts of the distributor cap, the overall resistance had increased and the coil warmed-up.to the extend that there’s no spark any more.
In addition, I found that (after only several hours of use) the contact points were showing some pitting and the beginning of a “crater and mountain” on the contact point surface, meaning my condenser/capacitor had gone. I replaced all that including a “new” old Lucas rotor arm.
I don’t know exactly what was the main reason for the car’s behaviour, but I suspect that the two causes together may have created an even bigger problem.
Thanks also to all who helped in finding a solution. In general I can say (at the same time answering some of your questions/recommendations) that my car is as original as (sensibly) possible. Fully “analog”, with (at the moment) even the original black B12 coil dated 1 54, standard HT leads, original Champion plug caps, etc. . See also the attached picture of the engine compartment. Many of the Lucas parts in the engine bay are still the original 1954 parts like Regulator, Fuse Box, DB 10 Relay, FL3 flasher (although with FL5 electrics inside), Trico washer system, etc…
Great…! the gaps, increasing resistance, but not so large as to have no flow, heated the coil AND the points problem-: the thread on this topic is a good checklist to have around. Nothing beats a great running XK !! Nick
