I have been told the only difference in a merc 20 horse and a 25 horse is the jetting in the carbs.Is this true?

 

Some motors are just de-rated to allow use in waters with a hp limit. Even a 1990 my Merc was available in an 18 hp version to comply with the less than 20 hp limit. There may have been a very minor difference. Some people just bought different decals. That sort of explains the old Johnson and Evinrude motors that were 9.9 hp when they could have been sold as 10 hp models, but the "less than 10 hp" was common on smaller lakes and in remote, protected lakes.

Dick