Well my friend, being where you live, MAYBE you use a fuel.additive like STABIL. If not, this is what i will be doing tomorrow with a briggs 3.5.....
First, im gonna dump the gas thats in it. (Give a hoot!DONT pollute) Then im gonna put in some fresh gas and slosh it around, then let it sit. While its sitting, Im going to HomeDepot to get a 16oz can of de-natured alcohol. Its by the mineral spirits , in the hardware store. When I get home, Im going to pour out the gas again. Then Im going to mix up a solution of 3:1, gas-de-natured alcohol. Ill put that in the tank, then run the engine. The alcohol will break up the gummy crap in the carb and end up burning it up. NOTE I ONLY use 12 ounces / 36 ounces..... THATS IT..... I have also used high $$ mineral spirits to do this with great success,cleaning a carb, tank, and spark plug of diesel fuel..... It dissolves the deposits and flushes them through. This I just did on a crf 80 my boss's son put diesel in by mistake..... DOHHH!
If your engine dont/wont idle/run right after that, you gotta open up the carb for a deep cleaning. Unfortunetly, Im no pro with BnS carbs so hopefully someone else will chime in on a "real" clean fix....
As a side note, one of my uncles is a master at everything he does. Engines,fiberglass work,body work, I mean this cat is a "genius". KBR has patents on a few of his "ideas" when he worked for them, making them BILLIONS, and screwing him...... Anywhoo, I was getting bad gas mileage in my 89 mustang 5 speed. He bougjt a can of de natured alcohol and put it in my tank,then filled it up. After that tank, my mileage went to BETTER than what it had ever been. And stayed that way. Since then, I "clean" my fuel system every 6 months this way, in all my vehicles. I cant/wont tell yall to try/do it, but I CAN say, I have NEVER had any problems with my vehicles by doing this. It has not done any damage to my knowledge. I had the mustang for 7 years, started the cleaning treatments after the first year....