Merc 3.0 won’t rev past 3000 RPM with load

[Philupay89]

Hey fellow captains! I’m having an issue with my 1993 Mercruiser 3.0L. Serial #: OF013552, with Alpha 1 Gen II Outdrive. When I’m on the water, boat starts and idles good. Idle is right around 650-700 RPM. Problem is, when I accelerate, doesn’t accelerate quick and won’t go over 3000 RPMs. Had the muffs hooked up today, and the boat accelerated nice and easily went over 4000 RPMs in gear. Boat is a Bayliner Sportsman 1952 open now.
I did have some work done at the beginning of the season:
-New SEI-116 Outdrive
-New starter
-Fresh tune-up. ie, plugs, wires, dist cap, carb adjusted.
Although outdrive is new, prop is old and pretty beat up. It’s a 3 blade 14x19. I can throttle up and plain out. I’ve actually got the boat to hit about 30mph with myself and fiancé on board, but still will not get over 3000 on tach. It makes it difficult to pull son on tube, and have more then 3-4 people on boat.
I did put some sea foam in just in case I had stale fuel.
If I recall correctly, the boat did reach close to WOT RPMs before I had the work done. Figured I’d start by changing prop, but wondering if I should go to lower pitch, but figured that wouldn’t accomplish that much RPM increase. I have read a checklist of things to check, but figured someone might have a quicker answer based on the work that’s been done recently, and my particular situation. I really appreciate any insight from you guys!

 

[Scott Danforth]

your motor would turn 5000 RPM running on one cylinder while in neutral

reving in neutral means nothing

check your compression on all cylinders
read your plugs, you may be running lean due to a crudded up carb

verify your tach

weigh your boat

a 19p prop may be a bit much for that boat and motor combo.

 

[Alumarine]

Either your tach is off or your MPH is wrong.
You can't go 30 mph at 3000 rpm if that's a 19 ft. fiberglass boat.
Are you using a gps to determine speed?

 

[Philupay89]

Thanks for the info, Scott! Now would the same apply if I run it in gear out of the water and it still accelerates well and climbs over 4000 on tach? However, I will be sure to do a compression check, and read the plugs.

 

[Philupay89]

Thanks for the response, Marc! No I am not checking speed with GPS, just speedo. It could very well be off, but I can tell I have to at least be doing 25 just based on spending time at speed on other countless boats. I can trim up while at WOT, and can tell I gain a few more mph as well.

 

[Rick Stephens]

Without a load on it, in gear and in the water, you are not testing anything by revving it up except whether something is ready to blow up. Hard on everything to do that.

What gear ratio is your new SEI drive?

 

[Bondo]

I did put some sea foam in just in case I had stale fuel.

Ayuh,...... Welcome Aboard,..... My guess is, yer Right,..... There ain't no "Fixin" rotten gas,.....

Hopefully the carb ain't fulla crap too,..... You don't mention changin' the gas filter,......

 

[Philupay89]

Thanks Rick and Bondo for the advice! Rick, only “on water” test I do is normally running the boat as I would. The test I did the other day was on muffs in gear, which I understand isn’t good for the motor, but figured it might be an indicator why it would rev out perfect and accelerate past 4000 in gear on muffs, but not with a load on water. Bondo, I have yet to check or change fuel filter. I’m going to change old beat up prop, then address fuel and timing.