Motor Rev Question

[ugadawg]

When I ran my boat a couple weeks ago, my motor began to run a lot more rpms than it usually does at the same throttle position, almost as if my prop was underpitched. I just figured I had hit one too many rocks and needed a new prop (see my previous thread). I got a new prop and took it out yesterday and had the same problem. The performance was a little better with the new prop, but not what it should be. I know the prop is pitched right because I have run the same prop on the boat for over 10 years. What I have noticed is a break on the carrier assembly.

prop.jpg (435.26 KiB) Viewed 2081 times

This piece has been broken in a couple spots for years with no performance problems. It only recently became broken all the way around (happened at Keaton of all places). Could this cause the prop to not be able to pull water like normal? Is there anything else that could be causing this problem?

[mjsigns]

Looks like you may have bent the prop $haft after the last rock you hit.... Put the engine muffs on and put the motor in gear ahead slow and see if the prop shaft is turning true. Hope it's not the prop $aft..

[ugadawg]

The prop shaft turns true. If it was bent I would also think the motor would vibrate when it is running in the water. The motor runs as smooth as it ever has, just acts like the prop is now underpitched.

[FUTCHCAIRO]

SOUNDS LIKE YA GOT ALL THE GARBAGE OUT OF YOUR HIGH SPEED JET AND THE FUEL IS FLOWING IN FREELY. JUST TO BE ON THE SAFE SIDE KEEP THE THROTTLE AT ABOUT 2/3 RDS, I NEVER RUN MY ENGINE OVER 2/3 RDS THROTTLE, I GUESS THAT IS ONE OF THE REASONS MY 1985 EVINRUDE STILL RUNS LIKE NEW, NEVER SKIPS A BEAT AND NEVER HAS.
PA
SEMPER FI

[Flint River Pirate]

Pa, I admire your restraint. I can't help but run WOT when I can. Whether running a 9.9 or 225 hp outboard. There is a lot of truth in limiting the stress though.

[Steve Stinson]

My guess would be that the exhaust is causing the problem. A lot of your exhaust comes out through the center of your prop. With the small break that you had before, not much was escaping in front of the prop blades. Now that you have a larger break, you are getting exhaust / air in front of the blades which is causing them to cavitate slightly.

If it were me, I would get both the carrier assembly and the skeg both fixed. If it hasn't damaged the internal parts of the foot, it is going to be a lot cheaper now, than later.

- Steve

[ugadawg]

I was thinking about cavitation but I had not thought of the exhaust coming through. Looks like a new carrier assembly is in order. I know I could use a skeg guard too. I've been through a few skegs on this motor.