Today, I repeated what I did yesterday. I started the truck, let it warm up for a few minutes, then revved the engine while in Park. The engine bogged and didn't want go beyond 2000 rpm. I moved it back and forth slowly while near the curb, put the transmission back into Park, and revved it again. Same result.
I then let the engine idle while I looked under the hood. I noticed that the glass-cased fuel filter right before the carburetor seemed a bit dirty. The engine was idling just fine. The picture of the filter is below. Interestingly, the filter before this one, also a see-through, didn't appear dirty. the only picture I took of it came out unclear so I will post another one next time.
I then revved the engine again, all the way up to 4000 rpm, and it did just fine. I then drove it around and again, no issues. What is it that seems to go away after some idling and high-revving?
I am planning to complete the cooling system flush this weekend, at which point I will install a new thermostat and check and adjust the timing from there (thank you Pugsy)
On a side note, I am still wondering about the fast idle cam and how sticky it seems. Could that be the cause? Perhaps it "moves out of the way" after the engine has been revved hard enough. I will try do this again tomorrow, and video the carb to see what happens. First, clean out the filter, of course.