I have a 96 75 horse merc--I notice the 75 and 90 are identicle in weight and displacement--the max rpm for 75 is 5250 and max for 90 is 5500--is the only difference in these motors caburator jetting
The reason the RPM's are different is the powerband the 75 runs better at 5250. There is a little more difference than just carb's. The port timing is the main difference
