[Yosemite Sam]

With what all has been said about this problem so far, if this were my tractor the next thing I would do is set the points properly (not because I think that is the problem, but because (I think we can ALL agree that this little engine will not run right if the points are not set right) setting the points is much easier/faster than rebuilding a carb, plus doing this will eliminate the problem of "fiddling" back and forth between carb and points later.

Once you find out that setting the points is not the problem... Remove the carb, take it apart, blow all the passage-ways out and SOAK IT AND ALL OF ITS PARTS IN CARB CLEANER OVERNIGHT.

The next day, blow it out again and install a fresh kit, adjust the float, put it back together and put it back on the machine.

Then, when you find out that it STILL isn't running right, remove the main jet and clean the little tiny holes in it out. The tube is hollow and it needs to also be clean between the upper AND lower holes (this is where your real problem lies).

Then put the jet back in, adjust it and drive trouble free.

Just in case this doesn't fix the problem then it is sucking air from someplace.