I have a old johnson outboard that is a 15 horse seahorse.I am not sure what year it is.It is model #fd-10,serial#1348448.I believe it is between a 1958-1968.It is in incredible shape but seems to be holding back a little bit on the power.I am running 50 to 1 oil ratio and unleaded fuel.I didnt know if a lead additive is needed or if it could be a carb adjustment or what.Any help on the age and problem?