460 carburator sticks

I have a 460 Farmall that I use to blow snow with. It runs perfect untill I use it for 15 minutes and the throttle starts speeding up by itself. The longer I use it the worse it gets. It almost gets the RPM's so high I'm worried about blowing the engine. I have to throttle back to idle and the engine then gradually slows down.
I have a new heat riser spring on the manifold and it appears to be workig correctly. The carburator turns all white from frost so I suspect it might be sticking somewhere. The govenor appears to be working right.
I covered the carb and guvenor with a piece of cardboard thinking maybe it was the snow mist freezing something up but that didn't help.
I would appreciate any ideas as I sure don't want to blow the engine as in warm weather it runs perfect. Thanks

It could be you need to take a look at the governor.If it was working right it'd never rev that high

I would pull that carb & check the screws that hold the throttle plate to the shaft.

The ice/frost you see building up on the outside of the carburetor is also building up on the inside around the throttle plate. As it builds up it starts to keep the throttled place from closing causing the high RPMs. This happens most often at temperatures near the freezing point. The higher the humidity in the air the worse the problem.

The change in air pressure from atmospheric to a vacuum occurs at the throttle plate. When the air pressure drops it absorbs heat causing the water vapor in the air to freeze at the point the pressure changes. When the incoming air temperature is too low to thaw the ice, when the engine goes under greater load and the vacuum drops, the ice continues to build up. As the speed increases the governor keeps the throttle plate closed more of the time which continues to build up more ice.

The solution is to get heated air into the intake of the carburetor or get the body of the carburetor warm enough that it will melt the ice. The heat riser valve is not real effective at preventing this problem because it is designed to increase the temperature of the fuel/air mixture after it leaves the carburetor. Heating the carburetor could lead to vapor lock problems at higher temperatures.

Making sure the thermostat works, covering the side of the engine to assure that the air flow from the radiator goes past the carburetor, and partially covering the radiator to get higher air flow temperatures past the carburetor can all help to correct the problem. The use of alcohol in the fuel can also help some. The alcohol in the fuel mixture can melt some of the ice as it passes the throttle plate.

Fabricating a stove around the muffler and running a tube to pull warmed drier air into the air cleaner inlet will help a lot. If you are using a snow blower you have alot of moisture getting sucked into carb causing the excess icing. I have seen this done on H models with real good results. Lot of carbureted auto engines have a setup like that.