Mercury 4.0 Gnat carb leaks fuel

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mr-steve-jones

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May 6, 2011
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Hello,

I am rebuilding a 4.0 Gnat 2 stroke that I bought as a non runner. It starts first time now, but leaks fuel from the carb after a few seconds.
I suspect the needle valve rubber has perished and won't seal against the needle with the pressure of the float. There is no spring on the float either, and one exploded view shows a float sping???

Does anyone in England sell spares for this carb?

Even if I can't get a Gnat part, does any other needle valve fit? The engine runs sweet otherwise, so anything I can adapt to stop the leak will be OK.

Any advice as always appreciated!

Thanks.. Steve.
 
sutor623

sutor623

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May 23, 2011
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Re: Mercury 4.0 Gnat carb leaks fuel

Yea you may be correct. Either that needle inlet or needle is worn out of spec, or that missing spring is preventing the float from finding the correct height. Float may be fuel soaked too. Did you rebuild the carb(s)?
 
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mr-steve-jones

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Re: Mercury 4.0 Gnat carb leaks fuel

Hello Sutor, Yes I stripped the carbs and cleaned, but can't rebuid as I can't get parts..
Used gumout / toluene to get rid of the jelly on the spindles and it ran perfect at second pull suprisingly :) Only problem when I shut it down it pours fuel.

I suspect the rubber has gone hard on the needle valve seat. Needle is not worn and looks floreless.

Float has a very feint sloshing sound inside and looks like it's been soldered at some point. Float spring is missing and float looks a bit deformed but the valve seals OK when float pushed up hard. I think the float will work OK, but rubber seal is dead after 34 years.

God knows why the float would be deformed.. Some people should not be allowed to use tools after seeing some bodges during this rebuild! Why is it so difficult to make new gaskets???

Just need an alternative solution to the needle valve please guys :)

Cheers, Steve.
 
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carholme

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Re: Mercury 4.0 Gnat carb leaks fuel

Would help to know the s/n/year of the engine.

Gerry
 
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mr-steve-jones

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Re: Mercury 4.0 Gnat carb leaks fuel

sutor623 said:
True.
Click to expand...

Sorry!

Serial No is 9304821 made in Belgium. Thats what is on the plaque, but I cant seem to find anything listed for these engines after 9256980. Guess 1978?

Carb has some casting marks of "6-76" (could be a date?) and "MERC CARB" on 1 side, and on the other top side it has the letters LMO.

Cheers.. Steve.
 
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carholme

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mr-steve-jones

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Re: Mercury 4.0 Gnat carb leaks fuel

Gerry,
It is a 4.0hp Gnat engine. 2 cylinder.
All those sites check out as only having the float still available...

Has anyone ever fitted a different needle valve and seat to the float chamber?
 
M

mr-steve-jones

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May 6, 2011
Messages
41
Re: Mercury 4.0 Gnat carb leaks fuel

It is definitely the Gnat 2 cylinder motor, despite the serial numbers being higher than those listed.

Carb pictured in http://www.crowleymarine.com/mercury-outboard/472.cfm is the correct one.. Definirely LMO 1 carb rightly or wrongly.
Surely there must be an alternative float needle valve that would screw in from some other motor, like the 40?
Everywhere that lists Gnat parts says "unavailable" and some sites don't even list the needle valve assy.

Cheers, Steve.
 
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carholme

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