If the propblem persists, and you've tried all of the suggestions listed here, I've got one more idea. Often, you'll get crud in the tank that will stick to the fuel pickup inlet in the tank. Typical symptoms would be a tractor that runs fine for a while, then starts to bog down. You might shut down to troubleshoot, and when you start back up, everything might seem fine again - until for no apparent reason it starts bogging.
This occurs because the crud restricts the fuel flow when it sticks up against the fuel line inlet. Then, when you shut down and the suction goes to zero, the crud falls off the inlet. It's a problem that can drive you crazy.
If this is the problem you're experiencing, the best thing to do is drain your tank, and while the tank is draining, reverse-flush the fuel line with compressed air. The air will blow the gunk off of the inlet in the tank, and will keep the fuel stirred up while it is draining (to keep all the debris in suspension). You'll be surprised at the stuff you see come out... Oh, and make sure you have your fuel filler cap off when you're blasting that air through the fuel line.