Snownut

I have a Briggs 8.5 hp Powerbilt OHV snow engine, and its better on fuel than my neighbor's 5.5 hp Tecumseh L-head.

I had the same problem with my snowblower.  I have an 8.5 hp Briggs engine.  Excess snow contacted the front of the hot engine, melted, and made its way onto the friction plate.  The drive would slip in almost any gear, intermittently. It would usually slip at the worst possible time, in deep snow or EOD crud.  I fixed the problem by taking a short length of fuel hose and cutting a slit its entire length.  I slid the hose over the lip on the back of the PTO cover and also added a short length of foam caulking backer on the front.  I reinstalled the PTO cover and have not had the problem since.  I am also located in Wisconsin, so I got to try out my fix a few times already.