Avon s400 with 25 hp merc 2 stroke goes to max revs at 1/4 throttle

[Tjbooker]

I recently bought an avon s400 with a 25 hp merc... she ran great for 7 outings and then all of a sudden after giving her 1/4 throttle she revs to the max and slows down..... what can b causing this??? Took the motor in its good... marked the prop and no slipping.... help!!!!

[ShinyShoe]

my head is shouting gearbox... when you say took the motor in its good what does that mean? Clearly something isn't right.

[doggypaddle]

could be the prop rubber cush drive hub spinning? do you have another prop to try?
I would have though if it was the gearbox it would be accompanied by crunching and grinding noises.

[Tjbooker]

Tried another prop... same thing.... marked my prop.... no slippage.... adjusting the height of the motor today... gonna try that

[jwatson]

Sounds like cavitation. Is the boat correctly inflated? Trim set correctly?