I put a new TSX 883 Marvel-S carburetor on my TO 35, ran it, and then found water in the bowl. Would that explain why it would not start?
Yes, water in the bowl will keep the tractor from starting. It doesn’t take much water to stop the gas from flowing in the gas line. The vacuum created when the engine is cranked pulls the gas into the carburetor where it is atomized, and mixed with air before going through the manifold to the valves, and into the combustion chamber. To get all the water out of the fuel tank your going to have to remove the sediment bowl and valve assembly, or remove the fuel tank, turn it upside down, and drain it.
Once you have the fuel and water in the tank drained, you need to look inside with a flashlight to see if there is any rust. If there is rust you may need to put some ball bearings and mineral spirits in the tank to slosh around to remove the rust. In severe cases you will have to take the tank to a radiator shop (they usually fix gas tanks too) to have it professionally cleaned, and possibly have the inside coated to seal it. mineral spirits should displace the water in the tank, but better, after the tank is flushed with mineral spirits, flush it with alcohol to remove the last trace of water.
Blow out the fuel line and flush the carburetor with mineral spirits then alcohol to get rid of the water. Better yet, take the two halves of the carb apart, and make sure any rust residue is removed. Spray carburetor cleaner, mineral spirits, or even spray brake cleaner work well to clean the carb parts. Use the straw with the spray nozzle to blow out the jets and the nozzle. Put one of the additives that remove water in the tank along with fresh fuel. Remember modern fuels have alcohol blended in them, and alcohol attracts water. If you leave a can of gas out with the caps off it will actually pull water out of the air.