2800 RPM at full throttle under load

[carl1150]

I have a 21' cuddy with originally a 140 hp mercruiser but previous owner installed 120 HP mercruiser. It's impossible to get over 2800 RPM with this boat in the water, has no problem revving under no load conditions. 2 adults and 3 kids approx. load is 1200 lbs. and only runs 15-18 mph. I have run the boat with just me and the wife and got to 25 mph and same RPM. I have installed a prop recommended by Michigan Wheel and no change. MPH and RPM were identical before and after prop change. I adjusted the points to spec. and reset timing to 8 degrees, fuel filter clean, haven't checked fuel pressure but doesn't seem to starve for fuel. I'm gonna take it back out this weekend with intentions of diagnose and repair. Boat doesn't get up on plane I suppose because can't get enough RPM out of motor. Maybe just a good fishing boat like this but everyone I've talked to thinks this should still get to 25 MPH loaded. I know it's not gonna be a speed queen but would like to be able to pull a tube so the kids will wanna get out on the water. Any ideas or suggestions would be appreciated, I'm gonna run a vacuum test and compression test on motor to determine the internal condition of motor but it runs well so I expect no concerns in this area. Thanks in advance for the help.

 

[Bondo]

Re: 2800 RPM at full throttle under load

Ayuh,... My 1st guess is, the Po also put the wrong ratio outdrive on it...
It should be 1.98:1...

Welcome Aboard...

 

[Don S]

Re: 2800 RPM at full throttle under load

I have a 21' cuddy with originally a 140 hp mercruiser but previous owner installed 120 HP mercruiser.

For one thing, even a 140 is a very small engine for a 21' boat, but you should still get the rpm if the engine is running ok, and you have the right prop.
Have you looked to see what the gear ratio is on the outdrive? I would have to bet it's a 1.50.
Look at the upper gear box on the port side decal. I should have the gear ratio stamped in it.

 

[elkhunter338]

Re: 2800 RPM at full throttle under load

Check the simple thing first. The tach is it reading right. If you get more speed with just your wife and less speed with heavier load but the tach reads the same you have a problem with the tach.
I had the same problem but my tach would not rev. past 3,800 rpms, but the engine sounded like it was reving good.
Tach appeared to read fine at idle.
What size prop are you running. Send me a PM I may not check e-mails for afew days being it is the 4th.
I run a 21' starcraft cuddy cabin boat with a merc 140hp. I have an adjustable 4 blade prop from westmarine and I think I have it set to 16 or 17 pitch, at 4500 rpms = about 28mph

 

[carl1150]

Re: 2800 RPM at full throttle under load

I'll take a look and get back with a post, thanks for the info. guys. I appreciate the help.

 

[carl1150]

Re: 2800 RPM at full throttle under load

I had the boat out yesterday and was able to get 3500 RPM out of it on calm water, 2 foot or less. Once the waves kicked up was bouncing around from 3K to 3500, I assume that may be normal due to the waves. Speed was bouncing from 18 to 23 MPH but on calm water was a consistent 21 to 22. Looks like a prop to get the RPM up into range. The boat never felt like it went up on plane, what kind of MPH do I need to see it get on plane? Appreciate the help, thanks folks.

 

[vandentr]

Re: 2800 RPM at full throttle under load

I've got a 24 year old 21' open bow Sea Ray (don't let that fool ya, she's HEAVY) with a V-8, 1.50 drive, and 21 pitch prop. Mine jumps out of the water at 25MPH and will hold plane down to about 20MPH. The max revs on the tach read right around 4k at WOT and the speed hovers around the 45-48MPH mark...