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MG MGA - Float bowl to carb body leaks

I sent the carbs (SU H4s) to be rebuilt and when I got them back and reinstalled, there was gas leaking from where both float bowls attach to the carb body. I got new rubber seals, smeared a light film of Hylomar HPF (Universal Blue?) on the new seals where they contact the bowl, carb body, and metal washer and reinstalled. I tightened the "bolt" down to where it stopped. Both float bowls still leak more than a little. I ran up to my computer and did some searching and found one mention of not tightening the bolt too much (how much is too much?), but could find nothing in several manuals, or anywhere else, alluding to that. Any suggestions, observations, insights....? Thanks.
L.R. deOlazarra

Check the metal seal faces for nicks or burrs and if you find then, remove them with a flat file.
Art Pearse

The thick steel washer on the bottom is not flat, but spherical. The concave side must be up. If you put it wrong side up, or hammer it flat, it will leak.

There is shoulder on the bolt (early) or stud with nut (later). Pull it up until you contact the shoulder (tight). The float chamber should wiggle just a little with a firm push, otherwise very secure.
Barney Gaylord

Thanks for the feedback. Art; no nicks or burrs, so ok there. Barney; that was it... the steel washers were flattened. Bought a new pair and installed concave side up; no more gas leaks.
L.R. deOlazarra


Do you know if the later style mount with the stud and nut arrangement should have a dished washer at the bottom? If so, then Moss is selling another bad part. The set I received has flat washers for top and bottom. I have an old set of dished washers but the inner diameter is a bit too large for the stud. Anyone sell the correct washers for the late style mount?

David Ahrendt

David, -- I don't know the answer, but I can clarify some of the parts differences.
See attached illustration.
Part numbers on the left come from the MGA 1500 SPL.
Part numbers on the right come from the MGA 1600 SPL.

Engine change point is not recorded, but apparently sometime during early 1600 production. I don't know why part numbers for the banjo bolt, old style cup washer and rubber grommet changed between 1500 and 1600 books. Float chambers and throttle bodies are the same with same female thread for the fuel port, so these banjo parts should be interchangeable as sets.

The bolt and stud will have the same top end thread, maybe slightly longer on the stud to accommodate the AUC1384 brass seal washer. The bolt and stud will have the same barrel diameter to accept the same rubber grommets. The AUC1389 washer takes up a little space on the barrel of the stud; the AUC1147 (or AUC 2130) cup washer takes up similar space on the barrel of the bolt. As such the barrels should be about the same length.

AUC1389 washer (used with the stud) should probably be flat, as it takes the place of the flat face on the throttle body to back up the top grommet. This will have the same ID and OD as the earlier cup washer.

AUC1388 washer will have smaller ID than the cup washer, because it fits over the thread and bears against the shoulder on the stud. I would have expected this to be a cup washer similar to the earlier one, but it looks flat in the illustration. If it is supposed to be flat, then perhaps the barrel of the stud may be slightly shorter than on the bolt.

As a practical matter, when you need to replace the rubber grommets it may be easier to R&R the nut leaving the stud in place, although I have never had any problem to R&R the banjo bolt. The real difference is that multiple R&R of the bolt may eventually wear the threads or shoulder seating area on the pot metal throttle body.

As a casual observation, use of the stud will lower the float chamber assembly by thickness of the brass seal washer plus hex shoulder on the stud plus thickness of the top steel washer. I don't have those parts in hand to say what that distance will be, but this will effectively lower the fuel level relative to the main jet. This maybe some of the reason why the 1600 and 1622 engines use different fuel metering needle than the 1500.

Barney Gaylord

Barney, thanks for the great info. After reading your response I looked around the web for info on AUC1388 and everything I found called it a flat washer. So, maybe Moss is correct. I guess I'll keep the flat washers and see if I have any leaks.
David Ahrendt

This thread was discussed between 27/01/2011 and 05/02/2011

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