Reduced RPM at WOT after tune-up

[mdean]

1991 Mercruiser 3.0L with Alpha One outdrive.
Replaced fuel pump, distributor, rotor, plugs, wires. Cleaned fuel filter and flame arrestor, rebuilt carb, running fresh gas. No improvement.
As stated by the previous owner, the onset of the reduced rpm's did not occur until the Gimball bearing was replaced. Is there anything in the process of changing the bearing that may cause this issue or is it strictly a coincidence?
Engine sounds great, no missing, no odd noises. Any assistance with this issue would be greatly appreciated.

 

[etech2008]

Re: Reduced RPM at WOT after tune-up

no not at all although I have seen two things that could explain the coincidence

1. The shop that did the gimble bearing may have mixed up your drive with someone elses with different gear ratio and two the tech in the shop may have damaged prop and replaced it with whateverc pitch was laying around

I have seen the outdrive thing twice the last few years.

 

[mdean]

Re: Reduced RPM at WOT after tune-up

From what I can tell by doing a couple of searches on the serial numbers it is the correct outdrive. Also we have tried 2 different props and the result is the same, actually pulling the same rpm's with two different pitch props.
At about 3/4 throttle we are managing to get to 3200 rpm, also it takes it awhile to get up to 3200rpm. If we push it to wot we actually reduce rpm's to about 2700.
Any ideas?

 

[stupidforce150]

Re: Reduced RPM at WOT after tune-up

check your coil my boat is doing the same thing i just orderd one be here in the afternoon ill let you know...

 

[stupidforce150]

Re: Reduced RPM at WOT after tune-up

ya coil my 3.0L merc did the same thing new coil all good .......

 

[mdean]

Re: Reduced RPM at WOT after tune-up

ok stupidforce thanks, I will changed the coil and see if it clears up.

 

[Todd157k]

Re: Reduced RPM at WOT after tune-up

after you replaced the ignition parts, did you look at the plugs? There are several things that can cause that problem (on the engine side), almost all of them show up on the plugs.

Don S has covered most of them here:
http://forums.iboats.com/showthread.php?t=295070

 

[mdean]

Re: Reduced RPM at WOT after tune-up

Update: Ok well we put in new plugs, reset timing, did compression test. Compression test showed 140 to 145 in each cylinder, specs show 140.
Will put boat back in water in a day or so to test performance.
Thanks everyone for all the advice.

 

[mdean]

Re: Reduced RPM at WOT after tune-up

Thank you all for the great advice. After performing all of the work mentioned in my last update we put it in the lake. The results brought smiles, running great, pulling around 4600 rpm's at wot now. Next is the speedometer !
Once again thanks to everyone who assisted me on this issue.