Posted by Owen Aaland on February 23, 2011 at 18:50:23 from (65.174.247.39):
In Reply to: 460 carburator sticks posted by Michael Schwartz on February 23, 2011 at 12:51:19:
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.
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