Our 30 horse does that in cold weather and I haven't had an answer for it, perhaps there is some truth to the crankcase vent hose issue. Ours has the winter kit on it to help warm the air coming into the carb so I've just assumed the engine is not running hot enough and sobering fuel. Part of the cooling fan intake should be covered up but I haven't rigged anything up to control the air flow but it stands to reason its getting way over cooled in extreme cold. Right from day one I had issues with the crank case slowly filling with fuel from another issue, because the float doesn't seem to have the ability to hold the fuel back when there is any pressure in the plastic fuel tank, just shutting the auger off and its sitting there, especially with a full tank of fuel and the auger cranked up higher so the tank moves higher above the engine, that was bad news. I put a ball valve in the line just after the fuel tank and turn it off when I shut the engine down and that solved that issue as it was happening winter or summer and the engine wasn't running when this would happen.
I did notice a neighbor that had what looked to be a winter front that somehow attaches onto the side of the engine and had flaps that could be opened to allow some air through. It looked very professional the way it was made and not sure if he had it made or its something that some company has created.