Posted by Tom Fleming on January 17, 2013 at 15:30:31 from (67.20.233.75):
In Reply to: ice on carb posted by Mlkgsmer on January 17, 2013 at 03:38:59:
Ok, let me help out here a bit. The action of the carb is to vaporize the fuel. So, let's talk about the physics behind that. Boyle's Ideal Gas law states PV=NRT. Translation: Pressure x Volume = Number of moles of gas x Universal Gas Constant x Tempurature in Kelvin. so, to keep things a little simpler, Pressure and volume are directly proportional to the tempurature if you consider that Moles and the UGC are constant.
So, let's put this into even simpler terms. If you change the volume (ie. going through the venturi) the pressure and/or the temperature MUST change. Now, add on top of that the heat capacity of gasoline, which is how much energy (heat) it needs to absorb to vaporize. So, you lower the pressure (vacuum), increase the velocity (vaporize and heat capacity of vaporization) and you absolutely lower the tempurature of the carb. Can't change that fact. The tempurature drop can be dramatic. Think about when you drain your air compressors. Water can turn to ice because of the pressure drop and the associated tempurature drop.
Last factor, how much heat is being transferred from the manifold. If you have a thick gasket between the carb and the manifold, heat is NOT being transferred at a high rate. Add it all up.
Bottom line, depending upon a number of factors, including abient humidity, your carb CAN ice on the outside. If you are getting ice on the inside, likely problem is watger in the fuel. I can go into more detail if anyone wishes, but this is basic physics/chemistry. almost all my carbs ice, except for the F12/F14 (sits on top of hot manifold), and the Regular (sits on front of a hot manifold and is brass). I could make snow cones off my F-20. The cub has frost on it all the time. Heck, I started the KB5 the other day, and it's carb iced up. Don't worry about it unless you are having ice inside the carb, then it's a different problem......
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