As inexpensive as they are, put a new primer bulb on it. I've had several go bad over the years. Had problems with upstream and downstream ends.

Now that you know the engine runs fine on an external tank, identifying the core issue is a pretty straightforward process of elimination. An easy way to test if the starvation is caused by a blocked fuel tank vent is to just remove the internal gas tank fill cap while underway, and see if it "solves" the issue. If no change, maybe Uboo's suggestion of a new bulb would be worth trying. I know you said you cleaned the tank out really well not long ago, but I think it's still possible that something is getting clogged in the pickup tube (probably because my cousin had the same exact problems and symptoms not long ago, and he found algae clogging up the mesh on the pickup tube). He actually took OFF the mesh from the pickup tube (after cleaning the tank and refilling with fresh fuel), as he has a clear bowl Racor water separator/filter, figuring that it would be better to capture crud in the bowl where you can see it, rather than trapping it invisibly inside the tank.

When I ran it off the internal tank I did open the fuel cap and it didn't do anything. The bulb is new or at least new 6 months ago but easy to swap out. When I get back home I'm going to inspect the pickup in the tank and work my way back. It's either in the tank, on the pickup or the filter so could be easy to fix.

I am going with the comment that maybe your pick up screen is plugged. Is this a older tank that was empty during your rebuild. Try blowing air through you fuel line back to the tank. If it fixes it you need to clean out your tank.

Yes I think it's in the tank. What sucks is that I have close to 60 gallons in there to fool with

Jorge- with what you have already eliminated, my leading theory is the tank like the piece of paper or part number tag on my dad’s Pontiac tank- it was intermittent. But easier things first. Replace the fuel bulb with an OEM bulb, not an aftermarket one like a mechanic ripped off mine one time, exclaiming get rid of this piece of crap

Did you inspect the fuel lines yet?

strick

Could it be that a lot of crud was loosened up and didn't come out. Maybe clogging a screen in the pickup (which I don't see what its purpose is) , or clogging up the fuel filter, which I assume is ahead of the bulb.

Some pickups have a check valve in the elbow on top of the tank - they can get screwed up easily.

Got back from New York today visiting my daughter. Plan is to work on it tomorrow afternoon.