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Fresh rebuild on the engine and carburetor.
Had some issues with the accelerator pump on the ETW1 carburetor. Sharp acceleration, the engine was falling on its face.
Took the top of the carburetor off and checked the fuel level and the pump plunger. The plunger that was in the carb was the aluminum one shown to the left with the four tabs. I manually inserted the plunger into its bore. There was an interference like it was tight.
The diameter of the aluminum vs. the round brass was nearly .030" wider. Switched over to the brass unit and the accelerator action was much better. Nearly no bog on a push of the gas pedal. The brass piston is rounded on the edge where the aluminum seems more flat. Anyone run into this issue?

I've seen this before and there's a thread around here or on one of the PW sites discussing the issue. I've been lucky that I had a few plungers kicking around and could pick one that had a good seal but low drag fit. The ETW1's I've used on my truck have always worked really well once they were properly adjusted.

I had the same problem with my accelerator pump. I cleaned and rebuilt my carburetor using a rebuild kit and the the plunger that came with the kit is the one on the left. It was very tight in the bore but I decided to give it a shot. The truck ran terribly with hiccups under load and completely no power. I drove back to my garage and switched back to the original plunger and it runs great. I thought about trying to sand down the new plunger so it moves more freely.

The addition of Marvel Mystery Oil to the gasoline may help the plunger slide better in the bore. Today's Gas with alcohol gives the plunger a hard time.