OMC 2.5L 120 HP Engine max RPM at WOT

[StarCraft78SS]

Hello everyone,

As the title states, I have a 1978 OMC 2.5L I/O and I'm wondering what the max RPM at WOT would be in a ball park? I'm currently getting about 2800 RPMs with a speed around 26 MPH. We fully rebuilt the engine and the carb is running amazingly.

I'm not looking to be speed boat but just wondering if there is something else we can do? I feel like 30's shouldn't be that hard. It's an aluminum boat with a 13 3/4 x 15 prop.

Any thoughts or suggestions?

Wondering if anyone has any ideas on what the max RPM should be on this engine? It's fully rebuilt 1978 OMC 2.5L engine which some call the iron duke. I'm wondering what I should be getting for RPM at WOT. I'm currently getting around 2800 to 3000 and about 25 MPH with it trimmed in what seems to be the best spot for speed.

I have a 13 3/4 x 15 prop on it currently.

 

[Alumarine]

For the same vintage mercruiser 2.5 which is the same motor WOT is 3900-4300.
If your tach is right, you're way off.
With that prop I'm thinking your tach is reading low.
Although I don't know your gear ratio so I might be wrong.

 

[southkogs]

It's actually a Chevy 153 I think. 120HP. 4,000 to 4,400 should be your WOT RPM. And with that prop and pitch, you should at least get the RPMs if not the speed.

 

[StarCraft78SS]

Do you know a way to test the tach?

 

[StarCraft78SS]

Interesting, any ideas on why it might not achieve?

 

[southkogs]

Is this a "new to you" boat, or has it performed better in the past?

If it's new to you, I would start by confirming simple stuff: gear lube is good, ballgears are in good shape, good fuel flow, good compression, carb is clean and adjusted ... etc.

 

[Alumarine]

Do you know a way to test the tach?

Connect another tach to the motor and see how they compare.

At the back of the tach you have there could be a selector for the number of cylinders.
If you rotate it back and forth a few time that can sometimes get them working properly again.

 

[jbcurt00]

Merged

 

[Bondo]

Interesting, any ideas on why it might not achieve?

Ayuh,..... Welcome Aboard,..... A bad tach, or the wrong gear ratio in the outdrive,....

 

[Scott Danforth]

Other things that lead to low WOT:

• Water in the upper gearbox
• Bad carb linkage (look at bellcrank)
• Improper point gap and dwell
• Improper timing
• Crud in fuel system
• Low compression
• Improperly adjusted valves
• Worn cam lobes

What is your compression numbers for your "newly rebuilt motor"? Should be about 135-140 psi

 

[StarCraft78SS]

Is this a "new to you" boat, or has it performed better in the past?

If it's new to you, I would start by confirming simple stuff: gear lube is good, ballgears are in good shape, good fuel flow, good compression, carb is clean and adjusted ... etc.

This is a new to us boat but we rebuilt engine. We will check out these additional items. Appreciate the information.

 

[StarCraft78SS]

Other things that lead to low WOT:

• Water in the upper gearbox
• Bad carb linkage (look at bellcrank)
• Improper point gap and dwell
• Improper timing
• Crud in fuel system
• Low compression
• Improperly adjusted valves
• Worn cam lobes

What is your compression numbers for your "newly rebuilt motor"? Should be about 135-140 psi

Thanks for the extra tips, I'm not totally sure on compression but we could run tests. We didn't personally rebuild it but he is reputable mechanic with friends with the family for 30 years.

• Water in the upper gearbox - will have to check this
• Bad carb linkage (look at bellcrank) - will have to check this
• Improper point gap and dwell - I would have to assume this is correct during the rebuild.
• Improper timing - Again during the rebuild i would have to assume.
• Crud in fuel system - New fuel lines, fuel tank, carb. We've adjusted the carb a couple times and had to get a new accelerator pump due to sputtering and dying out while shifting into gear.
• Low compression - Will have to test this out.
• Improperly adjusted valves - would have to assume is correct.
• Worn cam lobes - we had him inspect this during the rebuild.

I appreciate the insight. We will have to check most of these. And I suppose it isn't always safe to assume things were done correctly no matter who it is rebuilding unless yourself.

 

[StarCraft78SS]

I should add, on the last outing, not that I want to admit this but I did some checking for weeds and noticed there was tape over the exhaust port on the outdrive from when it was painted. Wasn't fully covering the exhaust port but I wonder if excessive exhaust back pressure has anything to do with some of the poor performance? Thoughts?

 

[Lou C]

something that fools a lot of people is a stuck or corroded anti siphon valve on the fuel tank....

 

[StarCraft78SS]

Wanted to give an update on what we found. We found the timing to be retarded so we reset that and also that the engine started to run much better with the air filter off. It's one of those metal style ones, so we cleaned the filter and it took 2 cans of carb/ brake cleaner to get it totally cleaned out.

Runs like a top now. We will have to get it out on the water and see if that improves our RPM's. I would have to assume so.

Has anyone here converted a OMC 120Hp to a electronic ignition and fuel injection?