This one is long and complicated, but I'm really stumped here. Any help would be greatly appreciated.

Here's the main problem, my 3w-140 B2F quits while inverted, at low to medium throttle.

It runs fine inverted at high throttle. The engine bogs-down in the air the instant a role is initiated at any engine or air speed. This does not seem to be an overheating problem, since it will harrier forever (upright only) and climb out of sight at full throttle.

I bought the engine used, so I don't know the history on it. I've put over 40 flights on the engine, but only recently encountered the inverted quitting problem.

There's more, the engine runs perfectly in the pits, but bogs-down and blows smoke as it revs-up on takeoff. This is not a lean hesitation. It seems to be related to the forward motion. The plane has no significant speed when the hesitation starts (less than 5 mph). It only reaches full power after it is in the air. It revs to full power, without hesitation, in the pits. This happens hot or cold every time the engine is allowed to idle-down for several seconds on the ground. The bog-down at throttle-up does not happen in the air.

The fuel tank pick-up is free, and the fuel lines are tight. I checked the carb filter.

The abnormal thing is a restrictive exhaust system. The front stack is cut-off and capped on each stock, 2-stack, muffler. The exhaust restriction could be the cause of all my problems, but upright flight and static ground performance are fine. The engine also runs very dirty, blowing black residue. More residue comes from the left cylinder.

Engine setup:
3w 30x10 prop.
Ams Oil 100-1 mixed 91:1 with 93 octane pump gas.
The static pressure port is routed into the fuselage.
The bottom of the fuel tank is about 1-1/2" above the carb.
Baffled Extra 300L cowl with open hole below spinner.
Flying from sea level and 65F - 85F.

Thanks,
Cliff