Motor revs but boat does not go

[trainwrecker22]

I am writing this for a buddy as he is not Internet savy. He has a 1987 Thompson runabout. It has a 4 cylinder 2.5L iron duke in it. A merchant alpha 1 gen 1 120. He put a new prop on it a 13.5 x 17 prop the motor will rev but the boat will not get on plane. It just plows through the water. We put a 13.5 x 15 prop on it and it's better but still sluggish to get on plane. I am not familiar with alpha 1 gen 1 is there a transmission or anything that can be slipping or isomething it just too much prop? Also it want to pull to the left like a son of a *****. Any help would be greatly appreciated.

 

[alldodge]

I would start with doing a compression test, need to check the health of it. The transmission is part of the drive and it's all gears, so there would be no slipping the way you might think.

The drive may have to high a ratio, should be 1.94 and if you have a 1.47 or such this will cause some trouble getting up. To find out what ratio it is you need to rotate the input drive shaft and count how many turns the prop shaft turns. It can be done while on the boat.

Remove the wires off the positive side of the coil (this keeps it from starting).
Put muffs on the drive and turn the water on
Put drive in gear by moving throttle forward
Mark the crankshaft damper
Have someone out side watching the prop to count rotations
Now jump the starter so the engine rotates.

A 1.94 ratio should show almost to crankshaft rotations for ever one full prop rotation