I find it odd that the engine would still spin like that without power. It sure sounds fuel related. Almost sounds like a slipping clutch, but you'd know if that was it, of course.

You don't by any chance have a soft rev-limiting distributor rotor, do you?

Sorry for all the replies. I am just throwing out all of my crazy ideas.

Is there such a thing as partial vapour lock? The engine still runs and has some power, just significantly reduced, which means at least some fuel is getting into the engine.

If the clutch were slipping, the rpms would climb without the engine actually turning faster.

I do have a rev limiting distributor, for 6200rpm. I can easily see redline when the car's cool, or in 2nd gear going downhill, etc. so it's for sure not the distributor, unless its limiter kicks in early when hot.

I'll definitely look at getting some temperature paint for the rotors and do some braking tests on the street. Not sure where or how I'll do the test, but I'm sure it will be fun!

Possible that heat is having some strange effects on the advance mechanism in the distributor? No advance would certainly pull your power way down.

If the brake(s) are dragging the rims will get warm. Pull up to a stop, and put your hand near the rim to check for heat. If you cannot detect heat that way, put your hand on the rim. If it is brakes, I could help since I am just an hour away.

An hour? More like 20 minutes. I'll check that - thanks.

JW is going down the mixture leaning path - his logic is always a good bet.

Paint is a pita and a coarse measurement. If you want to test rotor temps, a hand-held IR thermometer would be a better bet plus you could compare left to right for differences.

Think fuel.

If he has fuel starvation would the engine not run lean? Then it should get very hot. Surely that would also be noticeable. What about a bad CDI or coil?

My volkswagen did the same thing and an airplane will too! high revs , more air+ humidity in air= carb ice. and it goes away in a couple of minutes, wont show up when driving "normal" and it's hard to catch. just my 2 cents worth.

Interesting idea - never heard of it before. How do you cure it permanently?

There is a low pressure generated in the intake to draw in air; the air is accelerated through venturis and around the throttle plates which creates a further pressure drop. Reduced pressure results in reduced intake air temperature. If the air is moist and the temperature drops to at or below the dew point, you can get fog or precipitation in the intakes. If the temp is cold enough that precipitation can freeze and restrict air flow. You occasionally see the phenomenon when a jet engine is at high thrust settings and a cloud forms in the inlet.

Look at your intake air snorkel. On my '71 there (was) a hot-air riser that ducted hot air into the intake when the cabin heat lever was activated. The warmer air was to remove/prevent formation of ice in the intake. I suspect it is the same or similar on your '72. Sometime around '73 the warm air supply was moved to the driver side and made automatic.

Finally I'm able to update this. MSD 6200 CD box to the rescue - problem solved. Had a very happy day at the track this week! :D