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Old 07-16-2014, 09:27 AM   #1
Lt. JG
 
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Default lack of power

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 havenīt 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 dosnīt 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
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Old 07-16-2014, 11:35 AM   #2
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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?
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Old 07-16-2014, 02:47 PM   #3
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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?
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Old 07-16-2014, 04:42 PM   #4
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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
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Old 07-16-2014, 04:58 PM   #5
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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.
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Old 07-17-2014, 12:47 AM   #6
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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.
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Old 07-17-2014, 12:58 PM   #7
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Thanks I appreciate that. I'm eagerly waiting for the info to se if my engine is good or bad.
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Old 07-17-2014, 08:07 PM   #8
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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?
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Old 07-20-2014, 01:26 PM   #9
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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.
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Old 07-20-2014, 05:57 PM   #10
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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.
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