Paul;
I can go along with your diagnosis, but why does it run "like a sewing machine" for a period of time before it runs like crap? If it is a vacuum leak I would think the leak would be present at all times. The thing that I can't figure out is that it has had two carburetors {one old and rebuilt and one brand new) and the symptoms turn out the same. Almost sounds like some sort of heat related vacuum leak not associated with the carburetor itself. Let's see what the OP finds after going through your checklist.
SimS
You raise a very good point, which got me to thinking of something else. These old engines sometimes had problems with vapor lock due to the fuel lines being too close to the engine. It wouldn't hurt to check the fuel line to make sure it's not too close. The fuel lines were usually run through standoffs that bolted to something so that they couldn't get bent too close to the head. The problem was often at the front of the head where the line wraps around the motor to lead back to the carb.
Another possibility is that the carb is slightly warped, and as it heats up, the distortion of the metal causes it to begin sucking air in at the base.
The odd thing about his symptoms is that he initially describes what sounds like a classic vacuum leak, but then when he gets to Denver, he's obviously running over-rich. Two descriptions that are at complete odds with each other. That's why I suggested he may have more than one problem.
Jerry's suggestion is a good one. If Ed is available, he can probably sort it out pretty quickly.
The key to situations like this is to eliminate things one at a time. Once you have verified that something is
not the problem, don't keep going back to it again. That will drive you nuts.
68ironhead points to a little-thought of issue. Sometimes there are vacuum connections in places you'd least expect them, like the wiper motor, and people seldom even think of those or else they assume it's OK. Never assume anything is OK until you've checked it out and verified that it is.
Some of the advice he got from professional? mechanics is nuts. What does the accelerator pump have to do with a rough idle? Again, mechanics are just guessing. They clearly don't even know that an accelerator pump is only used during - wait-for-it - acceleration. Sheesh.
Here's some things to think about. The problem occurred
before he changed the carburetor. "would only run if it was taking on a lot of fuel"
After he changed the carb, he had the exact same symptoms - "Will not hold idle without gas"
So, changing the carb changed nothing. Therefore, it's not the carb, although changing the carb could have introduced a second problem. He doesn't know if the plugs were fouled in his original location, because he never checked them until Denver. So, what sort of condition could cause the engine to need additional fuel to keep running yet fouls the plugs from too much fuel and is
not related to the carb? Any time a motor needs additional fuel just to keep running, it's getting too much air. But, how does that foul the plugs?
If it were me, the first thing I would do is change the plugs. If it runs fine, then you know the poor running is caused by fouled plugs. Then you just need to figure out what's fouling the plugs. Obviously, it's too much fuel in the compression chambers. There are several things that can cause that; maladjusted carb, too high a pressure fuel pump, blocked air passages screwing up the fuel/air mixture, carb adjusted to overcompensate for a vacuum leak, a weak spark that prevents all the fuel/air mixture from being consumed.
Of course, all of this
assumes that he's buying the right parts to begin with.
