lack of power

[Fippe]

Hello,

I have a mercruiser 5.7 2bbl mercarb thunderbolt 5 wo knocksensor in a maxum 2400scr from 97.
I have rebulit the engine after a blown headgasket,
when i bought the boat it only rew 400-4200rpm and I thought that after the rebuilt it would work fine, but I was wrong it still only rews 4000-4200 on wot
to make shure that the ignition works fine i hade a mechanic too adjust the ignition and after that still no difference,
now I also have changed the fuelfilter, rotor, distributor cap and cables,
when I did that I noticed some rust on the sensor in the distributor, cleaned it up a bit an still no difference.
the only thing I haven´t done now is rebuilding the mercarb and looked att the antisyphon valve.
the engine runs fine good trottle response and always starts fine,
when i changed the fuelfilter i noticed that it dosn´t smell as much as it used to do, when I look at it in a glas it looks fine, i use 95 octane gasoline (sweden)
is it posible that the antisyphon valve or the carburator makes the engine not have the power it use to have,

the propeller is a quicksilver 4blade 16p

[mmwjr]

Fippe the 2400 weighs 5000 lbs dry (no gas, oil, gear, ...). So I have to ask some question:

1. What drive and gear ration?
2. How many passangers on board?
3. Gear weight?

A 4 blade prop will give better low end performance but hurt the top end. It also loads the engine down more and with a 2 bbl may the reason the top end is low. You are close to the recommended WOT.

Does the boat perform well for you needs? If not what are you looking to achieve?

[Fippe]

I think it is 1,47:1
Last time it was 2 persons and normal gear,
When the boat was put in the water the crane driver told me my boat was around 6250lbs and that's with around 60-100L fuel, no water and all gear, he told me that the weight is not exactly due to lack of calibration.
I have five battery's in the boat they are quite heavy.

The only thing I want to achieve is to make sure that my engine isn't getting overloaded.
As it is for now I'm satisfied with the performance, just a bit afraid that the engine is working to hard and due to that will not work that long.

But if it's as I understand that the 5.7 2bbl original is performing in the lover end of the recommended WOT the two ways to go is to change to a 15P prop or change carburetor and intake manifold.
If I change to a 4bbl holy carburetor and suitable intake wouldn't that increase my hp and maybe make my boat perform little more?

[mmwjr]

Based on the gear ration I am assuming it's an Alpha drive.

Weight with fuel sounds about right.

dropping 1" of will increase WOT by 200 rpm. Try using mercruiser's prop calculator to see what it recommends.

Going to the 4 bbl with increase HP but not necessarly WOT

[Fippe]

It's correct alpha one from 97.
Why won't higher hp increase rpm at WOT?
If I get more hp wouldn't that make it possible to rev higher when the extra power gives the boat a extra push in the water.

[mmwjr]

Adding a 4 bbl intake and carb biggest benefit will be HP across the entire rpm range, it will give a slightly higher rpm range but it's main benefit will be at lower rpms, however unless you change the cam you will not get the full benefit.

A friend has the same boat as you with a 4 blade prop, I'll see what size it is and let you know.

[Fippe]

Thanks I appreciate that. I'm eagerly waiting for the info to se if my engine is good or bad.

[Fippe]

I've spent most of my day searching around in the forum and net to learn about propeller diameter and pitch how it affects rpm and used several prop calculation programs and got me wondering if my tachometer shows the right rpm,
At a rpm of 4000-4100 my gps shows about 32kn with the four bladed prop (2162p16 I think) and at 3400rpm I get nearly 23kn can that be accurate?

[mmwjr]

I was at the marina yesterday but my friend was not there. I did look at his prop and it is a Quicksilver 2182P16 and I looked it up and found it to be 15 x 16P 4 blade Al. I don't recall what power he has but the boat is a 2000 2400SCR, I think it is a 5.0 carb engine.

[Fippe]

Thanks for the info, if he has a 5.0 engine ain't that engine delivered with a 1,62:1 gear ratio
and therefor needs a bigger prop?

This forum and it's members are great.

Found a chart that recommends for my boats weight and length a 14 1/2 X15 or 16x13 prop
And another thread on a forum with a same boat same drivetrain same problem as me, he found out that the original prop 14 1/2X17 is the prop most suitable for the job, he changed to a four bladed and got a big drop in rpm.

I hope the problem is the same for me, next week I'm going to a propshop and buying a 14 3/4 x 15 hopefully that will make some change if not enough I go back from 4 to three blades.
I will give a report next week when it's tested and I can see the result.

Hopefully I don't need too chase horses any more.

[mmwjr]

I would think the 5.0 would have less torque than the 5.7 and would need less pitch to get the same rpm.

I was at the marina and my friend was out all day so I did not get to talk with him.

I like you prop idea selection. Let us know how it works.

[Fippe]

I've tested a 14,75 diameter and a 15P 4 blade and the rpm didn't change much.
The new prop has a smaller diameter and pitch but I couldn't get the rpm increase I thought a should, only about 100-150rpms more.
Theoretical I should have got about 400rpm increase.
Now I wonder if I do something wrong to get to the top rpm.

When I accelerate the leg is fully in and I give full throttle,
When boat is heavy loaded I use the Trim tabs to help her up in plane.
As soon as she reaches plane I push the trim tabs back up again.
After a while I reach the limit for what I can achieve whit the leg fully trimmed in.
Now to the point when I trimming the leg out, theoretical I would get a increase in rpm due to less boat in water and therefore easier to push the boat forward, but that ain't happening.
The engine rpm stays the same and the speed too.

I've been easy on the engine and maybe reduce the throttle a bit to soon,
But I have also read that I can run on wot to long and if I do that May result in engine damage.

What the heck is wrong, is it me or the boat?

[mmwjr]

So you went from a 4 blade 16 P prop to a 4 blade 15 P, correct? If so I would expect about a 200 increase in rpms which is what you said you got. What is the current WOT?

Running at WOT for long periods of time will shorten the life of the engine.

Try keeping the trim tabs lowered, I do and use them to balance side to side loads depending on number of guests.

[Fippe]

I also changed the diameter of the prop from 15 to 14,75 four bladed, according to the propshop I could expect about 400rpms increase, maby they gave me the wrong info,

When I tested the boat before the prop change I was alone maby 20gallon of fuel and normal gear on board the wot rpm was about 4000,
When I changed the prop and tested we had packed the boat for a three day trip and was a bit heavier than the first test, now I reached 4100 again.
I thought that this small change was because the boat was heavier than the first time, but I got a feeling that if the boat is so sensitive for change in weight some thing is clearly wrong with me or the boat,
I've read that some have more people on board and still reaches a good wot,

For what I understand a unloaded boat should reach the higher rpm in the wot recommendation, when it do a heavy load will still keep wot in recommended rpm,
If I just make the engine reach the lover rpm in the scale the engine will work to hard when she's loaded for a week on the sea.

If I change my prop again to reach the recommended wot of 4600rpm I would have to buy a prop that have a 12 or 13P prop and that feels like the wrong way to go.

[mmwjr]

Frankly I think a 4 blade prop is robbing the top end. A 4 blade give a better hole shot and low end performance however that extra blade is drag and will impact top end, additionally you have a 2 bbl carb so this also impact HP and torque that this engine is capable of.

What speed and rpm do you typically cruise at and are you happy with how it performs there? If you want better high end performance I would try a 3 blade prop.
I have the same boat but with a 5.7LX (4bbl) and a Bravo 2 drive and typically run at 3800 rpm doing 34 mph.

[Fippe]

Most of the time I cursing around in 7mph but sometimes I also cruising in 27-30mph but never faster than 32mph at this pion the engine revs about 3100-3500 rpm if I remember correctly this might be a bit higher with the new prop.
If the engine wouldn't take any damage of working with to much load due to wrong prop or lack of hp/torque I wouldn't care that the wot only is 4100rpm, but I want the engine to work in the right condition, hopefully that will make the engine not break down.

Maybe if I buy a three blade 14 1/2 x 17 that's mounted from the beginning it will work better,

[mmwjr]

I think the 3 blade prop will get you the correct WOT, however I do not think the current prop will cause any damage with the way you are running the boat.