02 225 ox66, can't reach full rpm's

Moose · #1 09-23-2012, 09:31 PM

Im running with a 17 pitch merc prop 3 blade. I've already replaced the oxygen sensor, fuel filter, and water separator. I just started running ring free and sprayed power tune into the carbs yesterday to free up some carbon build up. I'm only able to reach 4800 rpms. It seems like she moving at the right speed for the rpms she's pushing. I'm sort of new to working on motors so I don't know all the parts or names so try to bear with me. Any information would be greatly appreciated.

McGillicuddy · #2 09-23-2012, 09:47 PM

I assume you are running a 23'. First check your tach that it is set for a 6 cylinder motor rather than 4.
Then measure the height of your motor. Anti-cavitation plate should sit at least 1-1/2"above the keel while the two are parallel.
Finally what prop (specific as possible... make, model, diameter, and pitch) are you using. Sounds like your prop has way too much cup and rake, or the motor is way too low, or the tach is off.

77SceptreOB · #3 09-23-2012, 09:48 PM

What boat do you have? 23' SeaCraft?

Moose · #4 09-24-2012, 12:55 PM

I am running a 1974 Seacraft 23sf. I'm going to measure the anti cavitation placement, and the tach when I get out of work today. However, the prop is a mercury marine vengeance prop that reads 48 16315 17p. I don't know what any of the numbers mean except for the 17p at the end is the pitch. I'll post again as soon as I check the tach and the anti cavitation plates. Also I am going to clean the vst today.

Briguy · #5 10-01-2012, 08:35 PM

I just had the same issue on the same motor. It was the low pressure fuel pumps. They are pretty easy to fix and won't kill your budget. Lot's of good info on tht.

Fr. Frank · #6 10-05-2012, 04:21 PM

Quote:

Originally Posted by Moose

the prop is a mercury marine vengeance prop that reads 48 16315 17p.

First, the Vengeance is a bow-lifting prop, not a stern-lifter, and designed for light-weight hulls. That prop will turn low rpms until it begins to ventilate, then with a heavy hull, it will lose effective thrust. Pitch is about right, the rest of the prop geometry is wrong.

But start with your motor mounting height.

Then, for top end, go with a 3-blade SS.
For low-to-mid-range performance and maneuverability, a 4-blade is better.
A 4-blade will always provide more stern-lift than a 3-blade of similar blade geometry.