The 70-90 hp engines from 1987 to 93 are 71.1 c.i and then they went to 84.6 c.i. To make your 75 a 90 just install the old 90 carbs as this is the difference. The lower units are the same gear ratio and to check your old one(90) just pull the bearing carrier and inspect gears and bearing, that the easy way..