Carb Icing 

Last fall a friend and I were out on the Tanana when the weather was cold and humid. He was running an 18 hp Go-Devil and I had my 25. Both engines started to lose power and the RPM was steadily dropping. When I noticed black smoke from the exhaust we pulled over to the shore. As we decreased speed we found neither engine would continue to run below a nearly full throttle setting.

I have an oil temperature gauge installed on my engine and that was reading over 50 degrees below normal. I suspected carburetor icing and an engine running too cold as the problem. We took the handy mans secret weapon (duct tape) and taped up the opening between the air cleaner housing and the front and sides of the engine. When we fired them back up they had to draw preheated air from the engine and ran as strong as ever.

I have noticed in the past, on other engines, that these little four strokes won't tolerate a dirty, oily or wet air filter and humidity below 40 degrees is an invitation to carb icing. Now that you know the symptoms and the corrective actions you can deal with it.