Carb freezing up

To answer the question... it's called carburetor icing and is caused by the cooling from the venturi effect. It can only happen when there is enough mositure in the air. It's rarely a problem unless it builds up ice thick enough to shut off the engine. In that case, the carburetor will begin to warm due to heat transfer from the ambient air and conduction from the manifold and the ice will melt. The engine should be immediately restartable.

It should only happen in an engine that is not up to full operating temperature. If it happens to an engine that has been running 20 minutes or more, it's likely due to a wrong or missing thermostat.

I've seen people make intake heaters which are basically just shields that retain some of the air around the exhaust manifold and hold it next to the intake manifold. Others heat the actual intake air in the tube between the air cleaner and carburetor. But as you know, warm air = less power.

Having worked on nearly every carburetor made, the M-S carbs are my favorite. Reliable, easy to work on, respond to tuning very well. Like most carbs, you want to get the float level right.