turned it 180? uh oh...the small hole that the pump in the carb uses to get its pulses would be covered up not allowing the carb to pump its own fuel when you do that. I just looked at my manifold to be sure and it only has the vacum hole that allows the carb to fuction one way.
the throat of my zama carb is the same size as the original but the venturi is slightly larger. Your thinking right about a larger carb causeing running problems but only in relation to rpm. Your running that 20" prop so your rpm would be much lower that the 18x6 im starting with. the larger prop causes a lower air velocity at the carb and would make carberation poor with too large of carb. I will work up to a 20"prop but I dont think the zama carb will have any ill effects with that prop only because I dont think I've overcarbed it, but time will tell. I have found that just making small changes is the best way to find rpm without blowing the whole setup. rotate your carb back to its original mounting and see if you have and improvement. Piper