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It happens whether you're in motion or not. On the road it occurs in second gear (not yet in overdrive) at about 35 mph. In third gear (out of OD) around 50 or so. It really sounds like it occurs at about the same engine speed each time. When I rev it in neutral in the garage and look down the carb the main nozzles seem to be spraying fuel ok. When I look way down the throat I can see the throttle valves and they don't seem to open up completely until the last couple of inches of pedal motion, and that's when the engine starts to lose power and run rough.

Oh, and the fuel pump is brand new from Max Merritt. It was a little different than the original, but Merritt says it's correct for that car and it fit fine. I approached this incrementally, replacing one thing at a time to see what the affect would be and changing the fuel pump did nothing.

Perform a road test:
When the enige begins to stuble thenn VIGOUROUSLY PUMP the accellerator pedal REAL FAST to see is that causes the stunble to disappear or get worse.

Lets not forget to check the exhaust and the flap in the exhaust manifold. A plugged muffler will keep an engine from going into high speed due to back pressure.
Wes

I checked the manifold heat riser valve and it moves freely. I did consider the possibility that an internal piece of the muffler could be jammed in such a way that it restricts the flow, but I have no way of testing that hypothesis, short of removing the muffler. And I'm sure it's probably rusted on to the exhaust pipes so it would have to be a job for a muffler shop. It may come to that I suppose.

Well, I think it's solved. PackardV8 suggested on this thread to open the point gap to 0.018". The Repair and Tune_Up Manual for the 23,22 and 21 Series has a page that says the point gap should be between 0.0125 and .0175". The Tune-Up Specifications and Adjustment Chart for 1949-1950 says .017" for the IGT 4203 Autolite distributor that I have. When I replaced the points about a month or so ago I set them to 0.016", which was a compromise and a number I remember using for all the old eight cylinder cars I'd owned in the early '60's. So anyway, I thought what the Hell, I'll open them up to 0.018. I did so and ran it and it was a bit worse. So I figured if opening them up makes it worse, then what will narrowing the gap do? After all, the one tune-up guide said I could go all the way down to 0.0125. So I incrementally brought the point gap down and lo and behold it ran better! I finally settled on the lower bound (0.0125") and it ran about the way it should, although it was a little sluggish (but no rough running), but I suspected the timing would be off a bit due to the point gap change, and indeed it was. So I advanced it a bit, back to spec. I haven't yet tested it, but I think it'll be fine now. My question is, why is it so picky? I've never owned a car where the point gap mattered that much. If you were within a couple few thousandths of 0.016 you were fine. When I set the points I move the crankshaft by turning the fan and I'm pretty good at getting the point cam follower right on the peak of the lobe. So I don't think it was an issue of not setting the gap accurately.
Anyway, I'd like to thank everybody who took a shot at this, you were all very helpful and I appreciate your help.

Congratulations. Wonder if the distributor cam lobes or the rubbing block is worn and can't open the points as far as normal anymore so needs the smaller gap. That problem was covered in another Gus story when someone had forgot to place a bit of lube on the cam when changing points. Can't remember the symptoms that led to the diagnosis.

Good to hear, now enjoy!
Wes

Need to check for distributor bushing wear.

And in post #1 of this thread u indicate the plugs are "little sooty" and running rich.

Are they still sooty and running rich???

What is brand and number on plugs??