The importance of a clean air filter.
Your lawnmower engine needs air to mix with the gasoline vapor to create the internal explosion that drives the motor. All this is controlled by the carburetor. The amount of gas used is controlled by the throttle valve that you can often manually adjust at the speed controls on the handle. The amount of air used is determine by the internal vacuum caused by the piston moving down in the cylinder. It creates a vacuum that draws in the Air/Fuel Vapor mixture before compressing it for the spark plug to ignite and produce the power stroke that drives the engine. So old gas that doesn't vaporize well or a air filter that is dirty will affect how well the engine can suck in the proper Air/Fuel Vapor. This is why a clean air filter and fresh gas are so important to a clean running engine. You are probably getting fresh gas for your engine as you use it up but are you checking your air filter?