If the chainsaw runs poorly, the fuel filter might be clogged. A clogged fuel filter is most commonly caused by leaving old fuel in the chainsaw. Over time, some of the ingredients in the fuel may evaporate, leaving behind a thicker, stickier substance. This sticky fuel can clog up the fuel filter and cause the engine to stall or run roughly. If old fuel was left in the chainsaw, drain the old fuel from the fuel tank and replace the fuel filter.