I purchased a 1977 Johnson 9.9hp, a buddy said if you change out the carb you can turn it into a 15hp. Which I did. Talking to one of the old service guys at the local shop told me that it really isn't a 15hp. That those older ones only produced 12hp. That I need to find a 1984 or newer one that is really a 15hp. That they opened the exhaust porting to produce the extra ponies to get 15hp...is this fact or fiction?