i am running a tall transom gtr on 1651f4 excel tall transom, just put on mikuni and built homemade ehuast but it still seems like it comes out the hole slow almost like the belt housing is slowing it down, but it is above the bottom of the hull. should i put a shim under transom bracket to raise the transom height to 21" or leave it at 20".

Does it sound like its slipping to you?

Have you played with the trim any when takin off?

If your housing was dragging it would make a big spray between motor and transom

Try running the motor in different pin settings.

is prop new or used?

pin is in top hole only because i dont run much mud. i get a liite spray when taking off but no spray when running, tried trim down prob half way , prop is worn a little not much though

Just because your in the top hole and turning up more rpm font means it's running faster!! My boat does the same thing but runs faster and better all around on the middle pin!!