After battling high idle issues, and tracking them down, I'm now having another issue.

After long periods at highway speeds, when I take the offramp, and eventually disengage the clutch when stopping at the end, the engine almost always struggles and dies. This means that immediately upon disengagement, engine RPM's fall from 4000 to about 200 or barely registering on the gauge. It will struggle for a couple of revolutions there before eventually dying completely. It never backfires. It always starts right back up. Sometimes it will struggle for up to 10 seconds before dying. Sometimes it will struggle for longer at that low speed before finally recovering and returning to 800 RPM.

It seems to be a function of the amount of time spent at high RPM's. For instance. After a 30 minute freeway trip, I can be guaranteed 40 seconds of barely running at 200 RPM, sometimes dying, sometimes not. If I let it die and wait 15 seconds for a restart, it will start right up but continue to struggle for the allotted time. If I let it die initially and wait 3 minutes, it will start up smooth as butter. The struggling time seems invariant to how many times I try to restart.

If I drive at high RPM's for say 1 minute, it means I'll only have 4-5 seconds of 200 RPM struggling. The amount of time it struggles or dies seem proportional to the amount of time spent at high RPM's.

If I come to a stop and leave the car in gear until the last moment, slowing guiding the RPM's down, it seems to sometimes help with the dying, but not always. As soon as I disengage the trans, it will start it's struggle once again.

I can save it from dying by pulling the choke out an inch or so.

If I let the warm car sit for a few minutes, I can go out and start it right up with no choke. It will idle just fine forever that way. It only struggles after going to high RPM's and coming back down.

I've been through several manifolds and gaskets and distributors. I've driven the car with the manifold vacuum port totally blocked. The problem stays exactly the same. The carbs are newly rebuilt.

Car drives perfectly fine otherwise. No indicator of fuel or timing issues at speed. It has plenty of pull, etc.

Currently running 12 flats down on the dual SU's, 8 deg timing.

I have no idea what else I can check. Any ideas?