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Mercury 3.5 float adjustment?

northcave

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26 Feb 2010
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Bristol
With kind advice from this forum I've become fairly adept at dismantling and cleaning my carb in a Mercury 3.5. Anyhow although it guns and starts fine I have two issues.

1. When move at full throttle for more than 1 minute it starts to stutter and I have to throttle back. I'm assuming the float isn't set right and as the chamber of fuel empties it is then not allowing enough fuel in to catch up with consumption.

2. When I turn the outboard off but leave the fuel tap open it floods itself. Again probably float not adjusted and therefore not quite blocking the fuel off when chamber empty.

I've tried bending the little metal flaps so that when he float is up it pushes against the needle and when it's down allow the needle to drop a bit. I suspect I need to be more precise than this but wondered exactly if there was a way to know how much angle to set them at?
 

Spyro

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18 Jan 2003
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7,521
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Clyde
With kind advice from this forum I've become fairly adept at dismantling and cleaning my carb in a Mercury 3.5. Anyhow although it guns and starts fine I have two issues.

1. When move at full throttle for more than 1 minute it starts to stutter and I have to throttle back. I'm assuming the float isn't set right and as the chamber of fuel empties it is then not allowing enough fuel in to catch up with consumption.

2. When I turn the outboard off but leave the fuel tap open it floods itself. Again probably float not adjusted and therefore not quite blocking the fuel off when chamber empty.

I've tried bending the little metal flaps so that when he float is up it pushes against the needle and when it's down allow the needle to drop a bit. I suspect I need to be more precise than this but wondered exactly if there was a way to know how much angle to set them at?
There's no adjustment on the float. You say you have cleaned it. Is the chamber absolutely clean? No corrosion that would cause the float to stick? Your symptoms are exactly the same as I get when the float chamber is gunged up. The bowl fills but the float does not rise and fuel keeps pouring in. Have you had the needle valve out and is it back in the right way?
 

northcave

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Bristol
There's no adjustment on the float. You say you have cleaned it. Is the chamber absolutely clean? No corrosion that would cause the float to stick? Your symptoms are exactly the same as I get when the float chamber is gunged up. The bowl fills but the float does not rise and fuel keeps pouring in. Have you had the needle valve out and is it back in the right way?
The adjustment is on two small metal bits that can be bent. One pushed the float need up when the float floats. The other sets the limit the float can drop. I agree the latter could not be there but the other can be adjusted.

Bowl and float needle is clean.
 

Spyro

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Clyde
The adjustment is on two small metal bits that can be bent. One pushed the float need up when the float floats. The other sets the limit the float can drop. I agree the latter could not be there but the other can be adjusted.

Bowl and float needle is clean.
I know these could be bent but I would have thought they are factory set and shouldn't be messed with but if it does the trick then good enough.
 

northcave

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I know these could be bent but I would have thought they are factory set and shouldn't be messed with but if it does the trick then good enough.
Too late. I messed around with them a while ago before I got better advice. Needle is new but will test the float for a leak
 

thinwater

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12 Dec 2013
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Deale, MD, USA
I think the float is foam-filled. The only way to tell if it is heavy is to replace it. I've had that happen on cars, but not boats.

However, that does not address the WOT problem. I've had that too, same motor. Have you checked the tank filter? Do you have an in-line filter (if you do not, add one)?

Remember, it could be two unrelated problems. Outboards are like that.
 

duncan99210

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29 Jul 2009
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Winter in Falmouth, summer on board Rampage.
I've got the 2.5 version (same basic engine though). The problem I encountered was that the fuel tap wasn't allowing enough fuel to flow through it. It has a rubber/plastic mechanism which had deteriorated so that it blocked the flow of petrol. Thought I cleared it but I got similar symptoms, so I watched the fuel flow out of the pipe unattached to the carb: too little to my mind so I fitted a new tap which solved the problem.
 

RichardS

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5 Nov 2009
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Home UK Midlands / Boat Croatia
With kind advice from this forum I've become fairly adept at dismantling and cleaning my carb in a Mercury 3.5. Anyhow although it guns and starts fine I have two issues.

1. When move at full throttle for more than 1 minute it starts to stutter and I have to throttle back. I'm assuming the float isn't set right and as the chamber of fuel empties it is then not allowing enough fuel in to catch up with consumption.

2. When I turn the outboard off but leave the fuel tap open it floods itself. Again probably float not adjusted and therefore not quite blocking the fuel off when chamber empty.

I've tried bending the little metal flaps so that when he float is up it pushes against the needle and when it's down allow the needle to drop a bit. I suspect I need to be more precise than this but wondered exactly if there was a way to know how much angle to set them at?
The fuel height in the float chamber is controlled by the float acting on the needle valve and has to be set to an exact height otherwise the carb jets do not deliver the correct amount of fuel. You cannot just guess this but have to measure it accurately as set down by the manufacturer.

If you carb leaks fuel when the tap is left open, this means that the needle valve is not closing which could be due to a worn needle valve, a worn socket, an incorrectly adjusted float level of a damaged float which is not floating correctly.

You might have one or all of these problems but you can rule them all out by careful examination.

If the float has any fuel in it, it must be replaced.

If you can suck air through the fuel inlet into the carb with the carb inverted then the needle valve is not sealing properly.

The fuel height must be set correctly to remove that problem.

Richard
 

VicS

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13 Jul 2002
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44,712
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Home: Kent. Boat: Chichester
With kind advice from this forum I've become fairly adept at dismantling and cleaning my carb in a Mercury 3.5. Anyhow although it guns and starts fine I have two issues.

1. When move at full throttle for more than 1 minute it starts to stutter and I have to throttle back. I'm assuming the float isn't set right and as the chamber of fuel empties it is then not allowing enough fuel in to catch up with consumption.

2. When I turn the outboard off but leave the fuel tap open it floods itself. Again probably float not adjusted and therefore not quite blocking the fuel off when chamber empty.

I've tried bending the little metal flaps so that when he float is up it pushes against the needle and when it's down allow the needle to drop a bit. I suspect I need to be more precise than this but wondered exactly if there was a way to know how much angle to set them at?
1. sounds like fuel starvation. Check the tank vent . Check the fuel flow to the carb. i assume it has an integral tank, not a remote tank. Possible that the float is set too low

OTOH

2. clearly the carb is flooding which may be due to the float being set too high , or a worn or sticking float valve or damaged float

Check the float and valve , replace if necessary return the float height to its original setting and see how it goes.

You may find a manual with the float settings for your engine at http://162.144.28.33/lib/library.html other wise you will have to find a manual elsewhere
 
Last edited:

stuartwineberg

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24 Oct 2007
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Romsey, Hants
Just had similar problems on, 8 think, the same engine. It’s a tohatsu. If you need to replace the needle valve then definitely replace the seat. Also and excuse granny suckin eggs, check that the tail of the needle valve is in the runner on the underside of the flap that moves up and down with the float
 

northcave

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26 Feb 2010
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Bristol
Thanks all... ok just a follow up.

Took off the fuel tap and hoses and although appeared to allow fuel to flow freely it was in fact clogged up with grit. This would suggest no filter in built into the tank outlet. So I've cleaned that out and will get an in line filter.

Sadly I went out to test it and all was going well when my painter line fell off the bow whilst going full chat... and caught it in the prop, shearing the drive pin! :ambivalence:
 

mauaguia

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28 Jun 2020
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1
My problem with my 3.5 Mercury is that: It starts up very good, but after a while I see fuel coming out of the second little hose in the back of the motor. I f I let it go, the engine stops running after a few minutes. Is this related to the needle?
 

RichardS

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My problem with my 3.5 Mercury is that: It starts up very good, but after a while I see fuel coming out of the second little hose in the back of the motor. I f I let it go, the engine stops running after a few minutes. Is this related to the needle?
It's an old thread .... but you definitely have a leaky needle valve. A simple job to replace the needle and the valve seat although It could just be dirt stuck in the valve so perhaps worth checking first.

Richard
 

northcave

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26 Feb 2010
Messages
2,263
Location
Bristol
My problem with my 3.5 Mercury is that: It starts up very good, but after a while I see fuel coming out of the second little hose in the back of the motor. I f I let it go, the engine stops running after a few minutes. Is this related to the needle?
It’s easy to dismantle and clean the needle valve. you can also use a tiny wire from a brass wire brush to dislodge grit. I’ve had to do it a few times.
 

VicS

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Home: Kent. Boat: Chichester
My problem with my 3.5 Mercury is that: It starts up very good, but after a while I see fuel coming out of the second little hose in the back of the motor. I f I let it go, the engine stops running after a few minutes. Is this related to the needle?
As suggested check the float and the needle valve.

You dont say if its a 2 stroke or 4 stroke but I assume from your mention of "the second little hose at the back" that it is a 4 stroke. If that is so ITYWF that the valve seat is not replaceable as RichardS seems to think it will be

When you reassemble it make sure the needle is clipped to the float arm
 

VicS

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Home: Kent. Boat: Chichester
The words in bold are infantile and totally unnecessary. :rolleyes:

Richard
You told mauaguia that it is, "A simple job to replace the valve and the valve seat".
I am merely pointing out to him that if the engine is a 4 stroke the valve seat is not replaceable as you seem to think it will be

If it's a 2 stroke then valve and seat are combined as a single replaceable item

Your objections are pathetic
 
Last edited:

RichardS

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You told mauaguia that it is, "A simple job to replace the valve and the valve seat".
I am merely pointing out to him that if the engine is a 4 stroke the valve seat is not replaceable as you seem to think it will be

If it's a 2 stroke then valve and seat are combined as a single replaceable item

Your objections are pathetic

If that is so ITYWF that the valve seat is not replaceable as RichardS seems to think it will be
Your words in bold are indeed, pathetic as well as being infantile and totally unnecessary. :rolleyes:

Please stop this stupid vendetta which you have been waging for years and start behaving like an adult Vic.

Richard
 
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