Cat D4 tail seat

Has anyone had this problem. Second tractor to do this. The pony motor carburetor float needle with viton tip sticks in the seat. Can remove needle and seat and clean and was never dirty in the first place. Reinstall needle and seat and reinstall on crawler and works fine the first time you start it. Next week the needle is stuck again. No gas to carb. All gas lines are clean with no restrictions. What’s wrong? 

Thanks Don

Years back when ethanol appeared and some of the so called viton seats were not really viton it was a problem primarily with small gas engines that had foam floats.  Now days it is not much of a problem with a float sticking as most engines use brass floats with the small spring doodad that pulls the needle out of the seat when the brass bowl drops. My pony's have the brass floats with the spring doodad so never a problem.  Might look to see if your needle and seat have the spring doodad to pull the needle out of the seat when the carb is empty.