We had the same problem.  Rebuilt the MS carb several times, but still had the same problem. 
The problem is,   -  --  the new carb kits do not include the diaphram on the back of the carb.  It is a bad/difficult design to understand even for the best carb man you can find.  But that diaphram, if bad  will make the tractor run bad.  ( always loaded up on gas )  that little diaphram is expensive and not really easy to change, but it will make a difference in how the tractor runs.
It took several rebuilds, and a couple of different people working on this carb to finally find the problem.

Hope this helps