To big a prop or to much pitch will cause an engine to overheat and quit. Bast way is to tach. it Should turn between 7 and 8 thousand. Not enough oil can cause the same thing.

i think oil would not be a problem because theres oil coming out of the muffler. i got 15x13 apc prop. do you think that cutting it down a bit will help the engine rev? and if i cut, much much? would 1/8" make a difference?

Ernest:
Way to much pitch on the prop for an airboat. 13 in. pitch is more for speed. 6 or 8 in. pitch would be more appropriate. 16 to 18 in. dia. depending on rpm. 1/8 in. off tips wouldn't make a difference,1/2 in would, but still to much pitch.

hey!! guess what, i think the engine seems to sound better this time. i drill 2 3/8" holes on the canister muffler and the engine can now spin the prop at full throttle. so i guess this helps a bit.
do you guys have any idea how to quiet an engine because it seems to be screaming now, i dont want too much noise. thanks!

normally an exhaust muffler wiil be quieter with more small holes instead of 1 or two large holes. You mighit want to do some area calculating or the existing holes and try smaller ones on next muffler. I don't believe you gain any benefit by having much more outlet area than the area of the exhaust port on the head.