If you do not find a manifold leak you can richen the fuel mix two ways. One is raise the float level. I do not care what the specs say. The other is to enlarger the main jet with a drill. If you fuel has 10% ethanol in it this may be you problem as this fuel will need a BIGGER hole. If your carb has a removeable venturi be sure it is there. Years ago I work at an Allis dealer, a guy rebuilt a carb which was not his normal job, and he left out the venturi. The tractor would run just not very well.