MGA Forum

Having an issue with my front su carb on a 1959 MGA. Gas keeps coming out the back of the carb where it should attach to the manifold. It appears to be coming from where the needle goes into the jet. The jet and needle are new so not sure what is causing the gas to leak out the back of the carb.

Dave

Do you have the notched washer (#72) in place under the banjo bolt on the top of the float bowl. This notched washer is what allows the float chamber to be vented - and it is the route that any excess gas should go if the float chamber overflows (stuck needle valve or sunk float). Since you report gas coming up the jet the vent must be blocked. If the correct notched washer is in place, check for a clogged vent tube (kinked, bent or an insect nest).

See 'Fuel Overflow' here: https://www.mgaguru.com/mgtech/carbs/carb.htm

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Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

I never question Eric’s intelligence or good looks, but it sound to me like your fuel level in the float chamber is too high - but not high enough to overflow through the tube. Either you have a leak in the float ( so it is starting to sink) or your fork on the underside of the float lid is maladjusted would be my theories. If you shake the float and it sounds like a few grains of sand, it is leaking.

Thanks for your reply. I do have the washer you mentioned and the overflow tube is clear so as the next person suggested it may be the float level. I’m going to look at that.

Dave

Thanks for your reply. Float is not leaking so I’ll try to adjust the level and see if that works.

Dave

Dave, A quick way to check the float level with the air filters off is to turn the ignition on long enough for the pump to stop ticking and then feel for wetness at both jets. They both should be dry. If either is wet, raise the float level until dry. If both are dry but the bottom of the carb is wet, the float bowl attachment grommets may need replacing.

Silas

A last possibility could be the float valve (#80 needle) in the #2 post illustration sticking open. It should be free to move up and down in its housing (#80 large part). When the float lever is properly adjusted and the float moves all the way up with fuel in the float bowl, it should the needle valve should close. Bend the fork (#78) at the Y to raise or lower float level. The straight section of the fork should be flat against the base of the needle valve. If bent at the pivot, it can bind and stick open.

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Bill

1961 MGA
1955 MG TF-1500
1954 Chevrolet 3100

One last option from me - then I’m out of ideas -

Make sure the needle seat (part #2) is tight to the lid. I’ve seen parts with and without small washer. But if not tight, fuel can leak into carb even if needle & seat working properly.

David,

To narrow down where the raw fuel is leaking from, please do this:

Remove the three screws holding the front dashpot and piston onto the carb base and and gently set aside.

Now turn on the key and view the top of the jet. Is there any fuel flowing out of the jet orifice?

What do you see?

Don

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Look sharply after your own thoughts. They come unlooked for, like the morning dove upon your window sill, and, if you turn to your usual task, disappear; and you shall never find that perception again; never, I say-but perhaps years, ages, and I know not what events and worlds my lie between you and its return.

Ralph Waldo Emerson

Don

I did what you requested and I don’t see any fuel coming from the jet. I have been able to get all the leaks stopped but my fuel pump never stops clicking. This is my first time trying to start this car with a new engine and so far no go. I have spark on all four plugs. Looks more orange than blue. I’ve checked the timing. What’s weird is if I try to start it with the key off the starter and engine turn over well. But if I turn the key on the engine hesitates sometimes and it sounds like it wants to backfire from the muffler. I don’t know what the issue is.

Dave

So the overflowing gas issue is resolved? If so, most likely it was a sticking or dirty needle valve. What type of fuel pump do you have? The SU pump will tick rapidly for a few seconds until the float bowls are full then it should stop or slow to an occasional tick. Aftermarket fuel pumps (Facet is a common brand) click constantly - even when the float bowls are full. If you have an SU pump that doesn't stop clicking (and there are no external leaks) then it is likely there is a leak on the suction side. Or you're out of gas...

As for the current question, when you describe easy cranking ignition off but bucking, hesitation & backfiring with the ignition on that made me guess the ignition timing is too far advanced. The spark is occurring too soon - before the piston has approached TDC (Top Dead Center) and it's trying to push the engine backwards. Another possibility is spark plug wires incorrectly installed.

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Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

Hmm - I think the spark should be blue. Orange I think means it is too weak. Someone more knoowlegablle will confirm or advise otherwise I am sure

Yes, it should be a snappy blue spark. During cranking the system voltage will drop a bit (the starter motor is a big current draw). A slightly weak battery &/or less than perfect cable connections will also affect system voltage. Cleaning & securing all the connections (including the ground connections) helps to ensure all the available electrons can flow easily.

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Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

I do have an su fuel pump and I do have gas and a new tank so not sure why fuel pump keeps clicking.

You mentioned setting timing at tdc but I thought it was supposed to be set at 10 btdc. Am I doing something wrong??

Dave

7* BTDC (Before Top Dead Center) is the stock MGA static timing. 10* - 12* at idle speed (~ 850 - 950 rpm) is probably good. If it is significantly more than that then it is probably too far advanced.

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Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100