Last year's gas? Could be water. I have a Suzuki four and drained most of the old gas and refilled with fresh....had some intermittent power losses but attributed that to air in the system (disconnected the fuel line at the bulb and the hose leading up to the motor drained). I do know that outboards, even four strokes, demand fresh gas and if you drain the old and refill with new and still have the problem call Steve at Cathouse Marine. He is the man for all things boat.