A 650 is way to small for that engine. And if that is a 650 spead bore the problem is even worst!
The problem sounds like the front blades need to be opened so too far in order to get eoungh into the engine. And the blades are past the transistion slots.
Drilling holes in the front blades allows a small carb to run on a big engine. and that is what you have.
So, either drill or get a bigger cab. I think a 750 would still need some small holes drilled (0.070"). Get an 850 (bigger front blades).