I've this problem since I've had this tractor for about 8 years now. This tractor has new a fuel tank, new prebent metal fuel line, fuel tank valve, all new fuel filters, new 437 plugs, points, condensor, plug wires, carburetor rebuilt, 12 volt coil, new 12 volt alternator, complete new wire and key switch. Starts just great, Runs great for about an hour then it starts run rough and I have to run it with about 1/3 choke to smooth out until I'm done working the tractor. Today while plowing snow it started running rough again after about an hour and I had to once again run 1/3 choke until I was done plowing 30 minutes later. I've tried adjusting the carburetor but it has no effect on how it runs when it's running rough like this. I am out of ideas now because it's run like this since before I changed all the above. I never changed everything above because of this problem. Changing all the above was just to know I had everything new and hopefully have no problems while plowing snow. Any ideas would be appreciated.