carburetor

What can cause a carburetor to ice up?

Thank you so very much for the reply & info:) it was very helpful:)

yep, it's all part of fluid dynamic physics... Call it Bernoulli Effect, Venturi Effect, or whatever, the fact that airflow being drawn into the venturi must increase in velocity (to pass through a smaller area), and then slow down (after passing the venturi)... the change in velocity results in a pressure differential that draws fuel from the carbeurator bowl.  Combined Gas Law holds that pressure, volume, and temperature are all related, and that a change in any one, results in a change in the others.  In the case of a carbeurator, you see a substantial change in temperature just past the venturi, because pressure is lower... or as previously stated, there is vacuum.

My classroom example:  Place your hand atop or near the head of a running air compressor-  it will be warm, because it is compressing air (raising pressure), and the result, is a corresponding rise in temperature.

Take the end of an air hose, open the valve, and let air rush out.  The end of the nozzle will get very cold, because the air, under high pressure, is escaping, and expanding (filling a low pressure area), hence, absorbing heat... which is effectively cooling the nozzle.

Carbeurators can be adversely affected by this situation... you can have frozen throttles, and if there's moisture in the fuel, frozen fuel... but also, if there's enough ambient moisture, the carbeurator throat will start building up layers of frost, which constrict the venturi further, and compound the frosting (because this causes higher velocity and a greater drop in pressure).  This can result in substantial loss of power and serious engine damage... loss of mixture control, and ice-chunks coming loose and jamming an intake valve open, or even falling into the chamber and breaking stuff.  It is for this reason that piston aircraft have a 'carb heat' control-  exhaust gases or electrical heaters located in the carbeurator base warm the vicinity to prevent frost from forming.

Automobiles of the late '50's and on, and even newer tractors have a device called a 'heat riser', which is like a butterfly valve located in the exhaust manifold.  On most machines with a 'heat riser'  there's an outlet located on the ENGINE side of the riser, that goes to a 'stove area' at the carb base, and on the far side of the 'stove', an outlet area that goes to some point in the exhaust system downstream of the riser's butterfly. 
The butterfly valve is usually held closed by a weight, and opened by a bimetallic spring.  When the bimetallic spring is cold, the weight holds the valve closed, forcing exhaust gases through the carb base, and once warm, the spring opens the valve, stopping exhaust heat from passing through.  On V-type engines, the heat riser passage typically goes from one cylinder head's exhaust port, through a passage in the intake manifold (through the carb base) and then to an opposite exhaust port.  In normal running, exhaust tuning and timing results in little or no exhaust flow between the ports, but when cold, with the heat riser closed, there's enough heat flowing through to make it all run well.

And because of this, (and many peoples' lack of understanding), the reason why many older cars would get so ratsy when cold-starting... is because the heat-riser was stuck, broken, or rotted out, the bypass port was plugged, or the exhaust system was compromised (aftermarket headers, or crushed exhaust pipe) or an aftermarket carbeurator installed on an adapter, thus eliminating carb heat during warmup.

Allis tractor engines have a 'conductive' heat rising system-  the intake and exhaust manifolds are in close proximity where the carbeurator mounts to the manifold.  It's clearly not as complex as a mechanical riser setup, but once started, the fact that the exhaust is flowing will warm the manifold to alleviate icing.  It DOES want a little startup warming, which most consider to be unacceptable today, but back then, it somehow made sense to warm up the oil in the engine, transmission, and hydraulic systems before putting her to work.

Kritter, If you're having problems with icing, start it up, and warm it just a little off idle for about 15 minutes.  If you notice that it's not governing properly (either not responsive to load, or it's picking up speed) shut it down... and look into fuel quality- make sure you're not dealing with excessive fuel moisture.  Everything else should be resolved just by letting her warm up a bit first.