Problem - Merc 225 Offshore Carburated Motor

[Slipsinker]

Need some help. The motor runs fine in neutral, and revs up with no problems. Soon as I put it in gear it's like I'm dragging the dock behind me. Will only rev to 3K with top speed at 18 MPH at WOT. The boat is a 23' ProLIne WA. Any suggestions short of have a Merc mechanic look at it. Thanks for any help.

[Bob H.]

I'm not familiar with that particular engine but I believe your problem is sensor related, a limp mode to prevent damage. From what I've read most people seem to have problems with the temp sensor functions - if it's running hot the electronics prevent you from reving too high and cooking the engine. The rev limit is 3000rpms. Iboats.com has a good Mercury forum. If you're lucky it's only a sensor problem and not the module. My manuals don't cover that engine.

[WildeOne]

Revving in neutral tells you nothing except the motor will run. Running with a load in the water is the only thing of importance. I would start by eliminating fuel delivery problems. Check the suction side of the fuel pump back to the tank for air intrusion, obstructions, clogged or misrouted vent, etc. Then check the fuel pump. The carbs would be my last choice to check, clean and rebuild.

After you have done the above, I would check for strong spark in each cylinder, etc.

This link may prove helpful if you don't have a shop manual:

http://www.biggerhammer.net/mercury/factory_manuals/Service_Manual_Index.html

Good luck.

[mikie]

Is it sitting in the water? If so, has it been awhile since you used it? Heavy bottom fouling can cause the same symptoms.

[WildeOne]

mikie, bottom fouling would not prevent the motor from revving over 3K.

[mikie]

Will if it's severe enough. Drops the rpms on my buddies' 52' Sportsfisherman several hundred rpms with twin 8-92 Detroits along with the loss of several knots in speed. Sitting right in Kent Narrows, and only takes about a month of not moving to cause the problem. After pulling and powerwashing bottom, rpms and speed pick back up. Never under estimate a dirty bottom.

[WildeOne]

mikie, we're not talking several hundred RPM's here. The Merc should be turning ~ 5500 RPM @ WOT, not 3000. The boat's just barely planing or not at all.

[Little J16]

How is the compression,could have a bad cylinder.

[mikie]

To WildeOne:

It was just a suggestion to help isolate his problem, not something I need to debate someone about! End of conversation!