FX4lcx,
It seems you are loosing crankcase pumping action either through your carb or perhaps backplate.
My engines seem to draw well on no pressure, muffler pressure or case pressure.
They don't need a pump.
You suspect the perry, so I would start there - if it doesn't fit, either true it up, turn it down and make an adapter or get another carb as they go fairly cheap when they come up.
As a down and dirty initial check to see if you are heading in the right direction you might try to clean it well and use some silicone sealant. If it works and you are comfortable with it you might just leave it in this condition for a while
CP, the 6x6 prop in the thrust program reads 136 mph pitch speed and 2.24lbs thrust - this ought to be plenty thrust for any sleek airframe - and north of 3/4 hp[8D]