No plumbing problems here --
Mine ran great on the first two planes with stock muffler.
The Bisson pitts on the latest caused the fuel delivery problems.
I played with plugging exhaust but the pump is perfectly consistent full to empty no matter what the attitude.
Without a pump, it will never be.
I bought new about 10 years ago and it had the rear cover drilled and tapped for fitting.
Never heard of an upgrade carb.

Kirk