jrf,
You had me going there for a while because I just ordered that MVVS .49 this morning, and at $140.00 it isn't cheap. I called Morris and talked to one of their engine gurus.
I couldn't get it to accellerate cleanly in the midrange with the big carb, and it didn't have enough power to pull out of a hover with the small carb.
What you failed to say was the small carburetor was taken from a 40-size MVVS. Pilots looking to get more instantaneous speed out of small props have been making the carburetor swap. The standard size carburetor that comes with the MVVS .49 is designed to give the engine greater control with a larger prop, e.g., the 12.25x3.75. Instantaneous transitions/high acceleration is not important here. This is a 3D airplane and I'm looking for torque power, not speed power. If you overspeed this airplane you are guaranteed to be picking up the pieces from the flutter.
How old is your MVVS .49.? Morris tells me the engines being shipped today have a new-style carburetor that is optimize for the engine to swing a large prop.