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SU Carb

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liseco

Freshman Member
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I have recently acquired a MG Midget 1500, the SU Carb leaks fuel at the vent on the float chamber, so far the needles and floats have both been replaced. It is the rear carb if that makes a difference.

Any help would be appreciated as it is leaking badly and I'm afraid to drive it in case it catches fire.

Thanks
 
Have you also recently replaced a fuel pump? Fuel pressure needs to be low < 3 psi or you overwhelm the float seal.
 
In my experience, it's the newer Grose-jets... which can be sticky and cause the valve to stay open (and then float bowl will leak out the top). I think the ball has some sort of coating that can cause this.
I switched to Viton-tipped ones and all was well. I'm presuming you've done the proper leveling of the float.
 
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By the "vent" are you referring to where the gasoline enters the float chamber (on top) or where it exits the chamber? If on top, it could just be a loose hose clamp or a leaking float chamber gasket.
 
When you say you replaced the needles I assume you mean the mixture needles. The comments above, like mine below, are talking about the float valve needles which are not the same part. The float valve assembly (needle and seat) are attached to the lid of the float bowl.

Depending on the carb the vent can be a simple hole or a tube nipple so a drain/divert hose can be connected. If gas is coming out of the vent or between the lid and float bowl proper, that suggests the float valve is not closing properly. Like Mark, I have switched to using exclusively the Viton tipped needles after several problems with Grose Jets. The traditional metal-on-metal float valves work OK generally but the Viton tipped ones just seem to be consistently better and more reliable. If your fuel pressure is less than or equal to 3 PSI as Jim said and the float valve is in good condition (and the float level set properly) that should stop the problem you are having with the rear carb.
 
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