I have a 1960 Sea King (Model #GG8822A, equivelant to 5D17B Gale Buccaneer) 5 hp Deluxe outboard that seems to run great most of the time. Occasionally on my last two outings I began to feel a jerking feeling in the motor. It usually doesn't coincide with a change in the sound of the motor but a few times the pitch of the motor went up when I felt the jerking. After the first outing I replaced the plugs and lower unit oil. I've re-adjusted the low and high rpm fuel mixture with no effect. Don't know what to do next. Thank you. 