Briggs 20 hp - HELP!!!

I have a B&S V 20 hp engine that is giving me problems.

When it starts it, I put it in the start position and it fires, but runs very weak. Almost loping or surging. It also puffs out a bit of smoke not a lot, but some. It will die either after a few seconds or when it is put in gear.

Have been working on this problem a while and here is what has been done so far:

Rebuilt carburetor
New fuel filter
Cleaned out tank and gas lines
New gas
New coil/plug wires
New spark plugs
New battery

A few observations.

The fuel pump is working. Took the top half of the carb off and turned it on - gas does come out like a geyser.

The first time I put the carb on after rebuilding, it ran strong, turned if off, 4 hrs later, back to hardly running.

When I check to see if it was getting gas by taking to top of the carb off, I put it back and it ran strong for 10 min with no smake. Drove it around the yard. Then it died and was back to the same problem.

I have played around with the needle screw on the carb but that does not seem to make a difference.

This is a tractor that was sitting for a while and am trying to get it running. The Briggs model # is 460777-1279 and the engine year is 95.

Thanks for the time and all the help!