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Can you remove the air cleaner assembly one more time, and post photos of your engine compartment from all angles? Someone might spot something obvious.
You have no emissions, but do you have any vacuum hoses or thermo vacuum switches remaining?
Something is going wrong when the engine heats up. Perhaps a temperature sensitive vacuum leak. It doesn't have to be a hose, it could be the carb or intake seal.
And have you done a compression and/or leak-down test? Perhaps check the radiator for exhaust gas, too.
Well, I'm once again at wit's end. I'm now pretty sure the power piston is working properly. The arms were a bit bent, so I straightened them. The car seemed to run better for a while afterward, but not perfect. After a bit of driving it got worse again.
In addition to the new echlin ignition module I installed last week I have now put in a new echlin coil, cap, and rotor. No change to how the car is running.
When I start the car cold and let it fully warm up, it always drives flawlessly for the first few minutes cruising and wide open throttle both, then a couple miles down the road it starts sounding a bit off. Once it starts running rough it does so at any speed or RPM, steady cruise or WOT.
It sounds like when it's cold with the choke on it runs fine then? Have you verified that the choke is opening fully once warmed up?