Don't have "demon carburetion" but you might call it that

I swear, if I ever get my '71 Plymouth to simply run reliably, I'll be the happiest man around.

Back a couple months ago I was running a Holley 2280, and fighting stuck float issues. After lots of work I threw in the towel and rebuilt a basic late 60's low-top BBD.

That carburetor runs much better than the Holley ever did, but now IT'S acting up!

The first time I took it out on the highway I didn't take long to realize that everytime I try to accelerate more than 1/3 throttle, it goes over on it's face like it's gonna die.

Let off, it picks back up. I brought it back to the shop and revved it as high as I dared in Park and held it and I can't make it run out of gas or stumble.

The first thing that comes to mind is the power piston, but I thought that is spring loaded in the up position and drawn down by vacuum.

The fuel pump, lines etc are new. I'll dig into it, thoughts are welcome.