I guess the fuel tank is too high from the carbulator, though you say
you can not lower it.
There are engines that are not sensitive to the fuel level and that
are very sensitive to the position of the fuel tank, vary according
to carbulator and engine designs.
I think most 2 stroke engines and some 4 strokes belong to latter.
Lowering the fuel tank or using a fuel pump may solve the problem.
2 stroke engines have disadvantage in inverted running compared
to 4 stroke because of their lower combustion temperature.
That's my guess.

Tsutomu Mabuchi