no revs in a Mercruiser 5.7 carb.

[Alvaroe46]

Hi everybody,

I have a Maxum 2300sr. The engine is the Mercruiser v8 with alpha one gen 2.

The maintenance is up to date (exhaust manifolds, water pump, spark plugs, oil).

The engine sounds and runs well. In neutral it goes up to about 4800rpm, maybe a bit more.
But when I'm gearing up, at full throttle it doesn't go over 3400rpm. The top speed is 25 knots.
When the boat is loaded with 8 people it does not exceed 3000rpm and the speed is 15 knots maximum.
I think the propeller is 15.25x15 pitch or 15x17 pitch.

Any recommendations please?

THanks

[mmwjr]

If the boat is kept in the water verify the bottom is clean of marine growth.
Is the prop in good condition?
Are you sure all 8 cylinders are firing? And firing order is correct?

I would think a 19 inch pitch would be more suited to your setup but increasing pitch will lower top rpm.

How does it perform with only 2 people onboard?

[Alvaroe46]

Thanks for the reply,
The boat is painted (antifouling) and everything is perfectly clean since 15 days.

With two people on board and 3/4 tank it is 3400rpm and 25 knots.
Full tank 4 passengers same rpm and speed.
With 5 or 6 people it is harder to reach the maximum speed.

The propeller is new from 2 years ago, it is in perfect condition.
I feel like the engine can't take it anymore, but it should.

Thank you

Alvaro

[mmwjr]

Has the boat ever performed better since you owned it? I would perform a compression test to get the condition of the cylinders.

[Alvaroe46]

I bought the boat and a few months ago and now I'm using it.
It's worked like this ever since I got it.

I contacted the previous owner and he said it worked like that.On Friday I'm going to do a compression test of the cylinders...

I don't think it's working 100%.

But in neutral it goes to 3400 RPM
If there was a low-compression cylinder, it would fail every time, right?

Thank you

[mmwjr]

Yes if a cylinder has low compression it will always be bad. A leak down test can also be performed to determine if the issue is valve related and not rings.

This is why a sea trial and survey are a must when buying a boat.

[jrsick]

Low compression is definitely possible cause, and the service manual lists about 20 additional areas to check. I recommend you download service manual #24 from documents and look at section 1C-Troubleshooting. See Poor Boat Performance, RPMs too low, Carb malfunction, Engine Acceleration is poor, and Troubleshooting with a vacuum gauge. If you plan to get a loaner tool to check compression, you might as well get a vacuum gauge also. I believe your vacuum port is on intake manifold behind the carb (assuming you are carb not efi).

[Alvaroe46]

I tested the boat before I bought it and the previous owner told me it was because of the propeller and the dirty bottom.
The engine sounds good, it doesn't do anything strange.
Thank you both very much for the answers. I have the manual, I'll see what you say.
If you have any more ideas, please let me know.

But why does it do more than 3400rpm in neutral and with forward gear can not pass ?

Thanks a lot!

[Alvaroe46]

Originally Posted by jrsick

Low compression is definitely possible cause, and the service manual lists about 20 additional areas to check. I recommend you download service manual #24 from documents and look at section 1C-Troubleshooting. See Poor Boat Performance, RPMs too low, Carb malfunction, Engine Acceleration is poor, and Troubleshooting with a vacuum gauge. If you plan to get a loaner tool to check compression, you might as well get a vacuum gauge also. I believe your vacuum port is on intake manifold behind the carb (assuming you are carb not efi).

Thanks i will check the manual. And yes the engine is carb.

Thanks

[jrsick]

For why you can't get over 3400 rpm under power, any information here would be an educated guess. Low compression, fuel delivery, restricted exhaust, ignition problems, and prop issues would give you these symptoms.
Is the engine smooth at the 3400 rpm, or are you feeling vibrations? Is the engine running rough and missing at full speed?
You should remove your spark plugs to check compression. Be sure to look at the spark plug analysis discussion in the service manual.

[mmwjr]

In neutral any engine will rev easily. This tells you nothing since there is no load on the engine. As mentioned it’s time to work through the troubleshooting guide.

One note, have you removed the prop and verified its size? You may want to borrow one from a friend that’s around 14.5 x 19 to see if it makes a difference. A prop with too much pitch will impact WOT.

Another check is with the engine off advance the throttle to full, note the position of the throttle arm on the side of the carb where the cable connects. Disconnect the cable and see if you can open the throttle on the carb further. If so you cable is not adjusted correctly limiting travel.

[Alvaroe46]

Originally Posted by jrsick

For why you can't get over 3400 rpm under power, any information here would be an educated guess. Low compression, fuel delivery, restricted exhaust, ignition problems, and prop issues would give you these symptoms.
Is the engine smooth at the 3400 rpm, or are you feeling vibrations? Is the engine running rough and missing at full speed?
You should remove your spark plugs to check compression. Be sure to look at the spark plug analysis discussion in the service manual.

The engine at 3400rpm works and sounds perfect, it doesn't do anything strange.
I haven't even changed the distributor and spark plug wires. Today I'm going to look at these two things.
On Friday morning or Saturday I'll look at the compressions.
Many thanks


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[Alvaroe46]

Originally Posted by mmwjr

In neutral any engine will rev easily. This tells you nothing since there is no load on the engine. As mentioned it’s time to work through the troubleshooting guide.

One note, have you removed the prop and verified its size? You may want to borrow one from a friend that’s around 14.5 x 19 to see if it makes a difference. A prop with too much pitch will impact WOT.

Another check is with the engine off advance the throttle to full, note the position of the throttle arm on the side of the carb where the cable connects. Disconnect the cable and see if you can open the throttle on the carb further. If so you cable is not adjusted correctly limiting travel.

The propeller is 15.25 x 15.
And the carburettor thing, if it opens to the max, is one of the things I checked first.
Anyway i will check again the carb.

Thanks



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[donald]

I run a 14x 17p prop on my 95 2400scr 5.7 and I get 4600rpm and about 38mph.
I would drop to a 14x17p prop.
I had a 14x19p prop for a while and had the same problem.
Your 151/4 diam. is just like adding pitch to a prop.
I think you are over propped.
Just my 2c.
Don

[Alvaroe46]

Originally Posted by donald

I run a 14x 17p prop on my 95 2400scr 5.7 and I get 4600rpm and about 38mph.
I would drop to a 14x17p prop.
I had a 14x19p prop for a while and had the same problem.
Your 151/4 diam. is just like adding pitch to a prop.
I think you are over propped.
Just my 2c.
Don

Thank you very much, it is not the same model but it could be an example.
The thing is that today we have been looking at the distributor and it is timing.
Is there any way of knowing what the propeller was carrying when it left the factory ?
Sorry for my english im spanish

Thank you

[mmwjr]

Shrew might know what prop it came with from the factory. You can also try Quicksilvers prop calculator to see what it recommends.

Do you have the Thunderbolt V ignition system? If so see the manual as you need to install a jumper to set base timing.

[Alvaroe46]

Hi!!!

On the Mercruiser website, the propeller selector recommends a 16 x 11. I think it's a very low pich.

What do you think?

Thanks!!


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[mmwjr]

Originally Posted by Alvaroe46

Hi!!!

On the Mercruiser website, the propeller selector recommends a 16 x 11. I think it's a very low pich.

What do you think?

Thanks!!


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Very strange! The first number should be the diameter but an Alpha cannot swing a prop that big without hitting the anticaviation plate

[jrsick]

That is not the correct prop size for your boat.
First, let's be certain on what your current size prop is. Can you see the numbers either on the side or back of the prop? Let us know what they are and we can go from there.
Also, do you know what your outdrive gear ratio is? It should be either on a plate or stamped onto the drive.
I believe that the solution to your problem will be engine-related, but we should be certain on your outdrive setup. You never know what the previous owner did so it's best to check the basics.

[Alvaroe46]

Originally Posted by jrsick

That is not the correct prop size for your boat.
First, let's be certain on what your current size prop is. Can you see the numbers either on the side or back of the prop? Let us know what they are and we can go from there.
Also, do you know what your outdrive gear ratio is? It should be either on a plate or stamped onto the drive.
I believe that the solution to your problem will be engine-related, but we should be certain on your outdrive setup. You never know what the previous owner did so it's best to check the basics.

Hi!
I checked the cylinder compression today and they're all between 137 psi and 144psi...i think is not bad.
I've discovered an ugly spark plug wire and I'm going to change it.
All the spark plugs have come out in good condition!

The propeller reference, I can't see what it is. It's covered with paint. But I talked to the previous owner and he says it's a 15.25 x 15 Black Max. (ref 78116A45)

My outdrive gear ratio i don´t know wich one is i think this one is the reference
Transom serial number: 0F654103
Drive serial number:0F681637

thanks all for the help!!