We have a simple non-propelled walk-behind mower with a 4 horse 4-cycle (oil and gasoline not mixed together) Briggs & Stratton Engine. It has a lot of hours on it and is smoking out of the exhaust. To compound the problem, a helper grabbed the wrong can and fueled it with diesel. It wouldn't run on that. I changed the fuel and spark plug and cleaned out the carb. It runs now, but I think the smoke is worse. Is there an easy way to reduce the smoke? It's a cheap mower, but it's just the right size for trimming and we'd like to fix it.