I just purchased a 1984 Checkmate 17 foot Enchanter powered by a 1998 Mercury 150 cc 2 cycle engine. The boat is in very good condition and included a good galvanized trailer . The boat has a hot foot gas pedal and throttle go forward and reverse. The engine is a two stroke and runs well when the gas pedal is held down but seems to wants to stall when at idle . I thought I might change the six spark plugs and add some new gas and oil in tank to existing gaswuch is about a quarter tank any suggestions that perhaps I should do more maybe how to. Clean carb or any other additive ?