[Yosemite Sam]

Please know that I don't intend for this to sound "mean" but I haven't thought this through yet and it may just end up sounding that way...

We really need more information, what model tractor, what led you to believe that the carburetor needed to be rebuilt, did it do this before rebuild, how long have you had this machine, is this tractor "new" to you, did it just start doing this all of a sudden, have you been using it all summer long, what style deck, does it only do this with the deck engaged, how thick and tall is the grass you are cutting, does it run strong otherwise, does smoke come out of the exhaust or valve cover, does it start easily, when you turn the deck off does the problem continue?

Try to determine if this happens only when the deck is running.

Start and run/drive the tractor for awhile to see if it happens (deck not running).

If it doesn't, then engage the deck and cut some not overgrown grass.

If it happens, turn the deck off and see if it continues.

If it does then it is an engine related problem.

If it doesn't, hook up to something heavy and pull it around to see if it starts again (deck not running).

If it doesn't, then shut it off (the tractor) and see if the belts are "riding" in all the pulleys properly.

If they are, then try spinning all the pulleys (individually) by hand, to see if there is any excess "drag".

If you do these things and report back, we can get a LOT closer to helping you find and fix the problem.