has the gas tank been flushed and or the pickup screen cleaned? Debris in the fuel maybe collecting on the pickup strainer and slowly blocking off. Shut down, fuel backflows, knocks off debris. run again and collects again. 2 cents worth of ideas
SK

Jeff,

I had exactly this problem with my Hudson 8. It turned out to be the rotor in the distributor. As the engine ran, it had less and less power, until it would only idle. Apparently the rotor was allowing the spark to be more and more grounded.
Several times I had only tested the spark using the wire from the coil, and there always had been a good spark. Finally I tried testing the spark at one cylinder, when the engine had run for a while, and would only idle. There was almost no spark!
In the past, I 'd had two rotors fail, but they had failed suddenly, so I was surprised that the rotor was the source of this problem.
On another car, a 1948 Plymouth, a defective ignition coil produced exactly the same problem for me. The spark became weaker as it warmed up.
Perhaps a distributor cap could be cracked and cause this problem, but I have not experienced this.

Per

Per

Per, I will give the electrical a going over this weekend, I a spare cap and several rotors so I will see if I can get it to start and then swap the parts.

Scott, thanks I planned to drop the tank soon to work on the rear end so I will check the tank, I have checked the screen and things were good but good reminder when. It's down to make sure it isn't full of junk.

Have you checked that the fuel cap can breathe? If the hole is blocked the tank will eventually form a vacuum.

Thanks, good idea and easy to check.

Got a spare condenser? (You should always carry two or three!) Just for fun, try installing it.

I started there and no change, so I changed it again thinking I could have gotten a bad one. After that I checked plugs and wires and found a bad plug wire. I always thought it ran smooth until I started running on all 8, ha ha. But still had the issue, swapped coils with one I knew was good, then rerouted the fuel line to see if I could get it a little cooler. Then I changed the thermostat from a 175 to a 155, this seemed to improve things a little but probably just wishful thinking.

Cap and rotor swap tested didn't do anything either so I think I have run. The electrical course about as far as possible except maybe a wire to the coil or ground.

Jeff, have you checked the compression?. It sounds like the valves are almost open, NO LASH between the tappets?. When the engine warms up some or to complete operating temp., they are all off their seats just enough to cause a loss of power, but still kind of run. It is probably worth checking out. Make sure you use a blow gun to clean around all the spark plugs before removing them.

Jeff,
Any inline fuel filters, or have you added any?
One that is partially plugged did that to me...
J

Compression is OK at 80-100, when warm but I didn't check when having trouble, good idea.

I guess it will be a little while, I dropped the gas tank to check for junk and it is great, no rust on inside , no debris, buy I'm glad I did the gas line from the pickup line to the frame was a rubber line going over the exhaust pipe and when I dropped it is was so hard it broke in two. Once I get a gas tank put back up I will check compression and valve adjustment, I had used Walt's suggested cold gaps but maybe I am a little tight.

I really appreciate everyone's help, been a little difficult to run down but everyone's ideas are really helpful, thanks everyone. Trying to get it ready for taking my daughter and her friends to a high school sock hop this winter.