There has to be a slot or a hole in the perimeter of the disc or drum. Otherwise the pin on the crankshaft doesn't have anyplace to engage so it can turn the rotor. Oldlug is correct, at least my 3.5 has a drum rotor with a single slot in the perimeter. The crankshaft pin engages that slot and turns the drum rotor.

Your comments about changing carbs is confusing. Typically the front intake port should be above the crankshaft, not on the side. That will definately mess up the front rotor timing. My 3.5 is a rear rotor so I'm not that familiar with the front rotor engine. I don't recall ever seeing the carb on the side. If you simply put a rear rotor front case on the engine and didn't change the crankshaft then you have no drive pin to drive the rear drum valve. Pictures would be nice. It is not a carburetor problem. Some how you have screwed up the intake timing with your mixing of incompatible parts.