I have a 1987 165 hp mercruiser I purchased that hadn't been in water for 2 years I changed plugs points gas lines both fuel filters plug wires and it fired right up and ran great until 2 weeks ago it started back fireing at half throttle I thought bad gas so I drained the tank still back firing . I could get it wide open and it would miss a little so I checked the rotor it had a hole burn thru the center , I then changed the plugs again and the distributor rotor and points it ran great for an hour then stared backfiring again even worse so I pull the cap off and the rotor had broke again and chewed up the inside of the cap. When I pull up on the distibuto shaft it moves about 3/8 of an inch is this to much ? Possible bad cam? Help!