Percolation? Heat soak? Don't drive during summer??

DentalDart said
You co fused me with some of this... my car doesn't have EFI, its to expensive. Just a mech fuel pump (old one) and new avs2 carb. My fuel tank is vented. My timing doesn't advance past 36* as we installed a timing advance kit, unless even with the kit its able to advance farther than that when warming up? I have a new mechanical fan with a clutch on it.

Yes the sparking at the coil is weird and no one has ever seen it before? Lol.

I found one other internet post of a guy with constant on coils, but it was a coil pack fired corvette with efi.. So not much help with solutions.

The coil stores energy when current passes through the primary winding. When the primary is disconnected, the secondary picks up a voltage spike which gets turned into your spark. A constantly firing coil could be caused by any number of intermittent connections. The ecm could be failing, there could be a loose connection at your pickup or at the ecm. A bad plug to your pickup. A faulty ground somewhere associated with the distributor. Or even something in the ignition switch? It's hard to say without some testing. Typically a faulty connection won't stay consistently dodgy like that since the arcing going on will typically erode something and cause the connection to fail or at least melt.
Since it keeps doing it, I suspect the ecm is the cause but have no way to say for certain. It's just the one place I could see a transistor or something being right on the edge for a prolonged period, and when it gets warm enough it may just shut down or remain in the "switched off" state with respect to the coil ground.
But you say the spark is still present and constant on like that, even after the engine quits. It's possible the switching becomes so fast that the coil can't absorb enough energy and fails to jump the spark gap once hot..

This is one of those cases where it's not simple or obvious.. So it may be that parts swapping is the only "easy" way.

I'd wager that once this symptom is fixed, it stays running more than 20 mins.