Possibly old gas??
I have had the ethanol gas suck enough water vapor out of the air to make the gas almost non burnable. My son did not believe it, so I poured a few spoons full into a coffee can and set it out in the gravel drive. I tossed a piece of burning paper into the can and the gas hardly burned (it was like a smoky old road lantern of the 40's). After fully draining the tractor's gas tank and putting fresh gas in, it started and ran fine (better after running for about 10 minutes so it cleared the remains of the crappy stuff out of the carb).
Maybe this tractor has the same problem??
Paul in MN