I have had the same problem with my 3W100 and have tried venting the regulator reference point into the fuselage (no help). Also bought the 90 degree intake manifold and connected the regulator to the intake manifold (no help). It is better when you run the low end needle valve very lean, but then it sometimes quits.
I thought perhaps that excess fuel was somehow pooling up in between the carburetor and the cylinders since the carb is below the engine in normal flight. Then when the engine is turned over, gravity causes this excess fuel to be dumped into the cyclinders.
I admit this doesn't seem possible, but something is clearly happening here. Can anyone explain why changing the pop-off pressure fixes this? I believe the tank is pretty much in line with the carburetor anyway. And if the pump side of the carburetor is pumping properly, it shouldn't really matter.
Anyway, I may go ahead and attempt this modification. Others please let us know if this is the fix.