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Slow Acceleration Mercruiser 4.3 tbi

Old 07-15-2021, 02:05 PM
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Default Slow Acceleration Mercruiser 4.3 tbi

Hi all,
Have been scratching my head with this issue for days now. My Mercruiser 4.3 efi engine suddenly is experiencing slow acceleration. It will start fine, runs perfectly on idle , when on plane it will reach WOT Rpm and have very good top speed. Issue is that it lacks power to go on plane but when it does it will continue to accelerate perfectly. Sometimes when it will not manage to get on plane.

Boat weight distribution is always the same, Propellor has been the same pitch from new.
Compression is 120 on all cylinders
Fuel pressure steady on 18psi
New spark plugs 2021
New leads 2021
New ignition pick up the sender 2021
Injectors and Throttle body serviced and cleaned
Throttle body gasket new 2021
All Fuel hoses new 2021
Knock sensor new 2021
No fault codes
Battery full
Grounds all good
Alternator charging perfectly
IAC valve New 2021
Exhaust riser and Manifolds new 2021
Exhaust Shutters new 2021
Fuel pump new 2021
Fuel filter New 2021
Spark plugs running good fuel combustion color, not lean and not rich, just right
Distributor cap in very good condition
Distributor coil New 2021
Oil pressure sender new 2021
Changed propeller to the same pitch as it was always been from new
Water Thermostat new 2021
Engine temperature is always perfect at 160
Engine oil pressure between 50-70psi



Engine is TBI.
Does anyone have any ideas why the lack of power during acceleration and hard to get on plane?

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Old 07-15-2021, 02:27 PM
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Not really, but is your bottom dirty ? Hopefully not your mind ?

Also, what you are describing is typical of a fuel system with vacuum line on regulator, where regulator is faulty, or vacuum line is slit/cut/not attached/etc . Most tbi's don't, but does yours ?

Edit: double check timing too

Edit: "Suddenly" as in fine one day(s) and then not ?
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Old 07-15-2021, 02:34 PM
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Thanks for your input..
No Bottom is good conditions nd very clean , boat has been in water for two months and new antifouling before.

Yes Fuel system has vacuum line from TB to Fuel pressure regulator, Vacuum pipe is secure and not leaking at all.

Yes suddenly as it was fine on a Sunday and not the following day.

Will check timing to be sure as this I have never thought could be an issue.
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Old 07-15-2021, 02:37 PM
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is 18psi of fuel pressure typical on these efi engines............seems low in my world
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Old 07-15-2021, 02:53 PM
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Fuel pressure should be 30 psi at 1800 rpm
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Old 07-15-2021, 04:33 PM
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If you added fuel ⛽️ in the tank or an additive to the fuel that could be the problem ??
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Old 07-16-2021, 11:35 AM
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Fuel is new with no addivitves and also tested engine running from another small outboard tank with fresh clean fuel.
Engine works perfectly in all rpm ranges , just very poor acceleration and won't plane on load.. if load is removed boat will plane and ride perfectly.. Issue is that load was always the same and this reduction of acceleration happened suddenly .

Cannot understand..
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Old 07-16-2021, 11:37 AM
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Originally Posted by AllDodge View Post
Fuel pressure should be 30 psi at 1800 rpm

That what I was thinking too, but according to specs I found on the net for my engine it should run between 9-12psi -- 18 is max..
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Old 07-16-2021, 11:53 AM
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what boat is this, looks like you have approx 220 hp in the perfect world ? how much weight do you have to remove to get it to run
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Old 07-16-2021, 12:05 PM
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Originally Posted by F-2 Speedy View Post
what boat is this, looks like you have approx 220 hp in the perfect world ? how much weight do you have to remove to get it to run
It's a Rio 700 Crusier Cabin. Yes, put a lot of work and new parts into engine.


I have to remove about 80ltrs of freshwater to plane.. If I load with the same amount as we usually do every weekend it will not plane.. The boat is lacking power on acceleration only, when on plane it runs beautifully .
My only concern is fuel pressure as I'm not sure if 18psi is correct.

Last edited by KennethF; 07-16-2021 at 12:14 PM.
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