my boat idle fine some time but when i put it in gear it stalls and when i do get it going and i have it a full speed it does not seem to want to go fast at all like a little faster then idle speed. it's a 3 Cyclider 90 hp Yamaha 1985 motor on a 17 inch hual. changed the plug and oil thats all i have done when i bought the boat it was sitting they sad the changed the stator and the propeler. what can it be?