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MG MGB Technical - Twin SU carbs leak fuel

Ok, this sounds like a fun first project. I just brought home a 74 B Roadster and the Carbs seem to leak fuel into the canister and out the bottom of the car.

The car runs, but does not idle. The fuel leak is not small, as it seems like it's just going straight form the carbs to the canister.

I have one opinion that told me that it was a stuck jet (?) in the carb.

Where do I even start to find the problem? I have a BL manual on its way but I'm really compfortable with the B yet.

Thanks for any help.

Chuck
ChuckE


If the petrol is coming out of the overflow pipes , then its the float chamber valve in the top of the carb.
I've found mine play up when the car has been stood through the winter. You must be care full when cleaning this valve you do not disturb the actuation lever ( spring )It is important that both carbs have the same height of fuel in their chambers. Read the manual carefully.
I found the best way in the end was to buy a complete set of seals valves, sets , needles etc.When done the car was so much better improved......and on petrol too
Ray

Where can I buy a rebuilt kit besides Moss?
ChuckE

Chuck,
Since you live in Anaheim, try British Motor Service at 521 Katella Ave., Orange. Their phone is 714-997-1122. They carry Moss parts.
You probably have HIF4 carbs and the jets are accessed through the bottom of the carbs. Sounds like one or both of them are stuck open. A complete rebuild kit will provide all you need to restore the carbs. The job is fairly simple with the proper instructions.
Cheers,
Gary
Gary Cash

Thanks Gary.

"The job is fairly simple with the proper instructions"

We'll see about that. The last time I took an SU apart was 10+ years ago. Let's hope I have better luck than I did then. ;-)
ChuckE

What kind of a pump do you have? Most after-market pumps have too high a pressure which can force the float valve(s) open and flood. You could also have dirt in the fuel system which would contaminate your new valves. I would run the engine with the pump disconnected until the float bowls empty, then reconnect the pump. The rush of fuel through wide-open valves should flush anything away. If they still leak then yes, replace them. If they then don't leak, but do so again soon, and you have a fuel filter, replace it. If no fuel filter then you would need to think about flushing the tank and pipes.
Paul Hunt

I don't know what kind of fuel pump it is, but the PO did say he replaced it. Before I go and start buying stuff, I'll have to check what kind of pump it is and try what you suggested.

Thanks Paul.
ChuckE

The original type is an SU that chatters when you first turn on the ignition then slows and stops as it pressurises the system. Thereafter is just clicks occasionally as fuel is required. Some aftermarket pumps chatter at the same rate all the time, somw whirr all the time. But if you have a fuel leak then even an SU will click more frequently than it should, But there should still be enough difference between first switch-on after standing overnight, for example, and the constant clicking even with a fuel leak, for you to tell.
Paul Hunt

SO far from the advice I've received, the Carbs are definitely stuck open which explains the fuel leak.

NOw the question is why?

First, it could just be a stuck needle in the SU carbs. Or it could be stuck open due to too much pressure from the fuel pump.

So, before I start taking the carbs apart, I think I should check the fuel pump to see what kind it is and see how much pressure it is delivering the fuel at.

Also, to disconnect the fuel pump and see if the carb no longer is stuck open. IF disconnecting the fuel pump cures the fuel leak, then I can assume it's the fuel pump that's the problem and not the SU carbs.

How's that sound?
ChuckE

*All* pumps supply *some* pressure. Too much pressure will overpower the valves in which case the pump *is* the problem. But *all* pumps will 'cause' a leak if the problem is worn or float valves or dirty fuel. Worn valves can be replaced. But if dirt is the problem then *that* is what has to be corrected.

But you are correct in saying that the first thing you must do is to find out what pump you have. If it is the proper SU then move on to finding out whether there is dirt in the valves or they are just worn. If you have the wrong pump and it is supplying too much pressure then you can replace the pump, or fit a pressure limiting valve between pump and carbs. Even then you may still have worn and/or dirty fuel so may still get a leak.
Paul Hunt

This thread was discussed between 14/04/2002 and 17/04/2002

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