This is a part we can all do without. It doesn't even fit a standard MGA H type carburetor, and I wouldn't ordinarily mention it, but the question keeps popping up.

During the production period for MGA Twin Cams there was a problem with a lean running condition which would burn pistons. One unsuccessful attempt to fix the problem was to install thin spacers to raise the cover on the float chambers, effectively raising the float and the fuel level to make the carburetors run intentionally rich. As this did not solve the problem at hand, the special part was quickly abandoned and was never a part of any production MGA.

Apparently someone had seen documentation of this part, or had seen it installed somewhere, and the reaction was, "Hey, another racing part we can sell for ten times what it costs to make". The part in the picture was fabricated to fit the HS type carburetors, which not coincidentally happens to be the big runner on pre-1972 MGB cars. It seems to be lost that they did not make the part to fit the original intended application of the H type carburetor, so this part does not even have any historical significance.

If you install this part to raise the float cover, you then need to lower the float level setting to return to the correct fuel level for proper tuning of the carburetors. That adjustment defeats the original intent of the spacer, which means it does nothing except to make the float adjustment specifications non-standard. It also takes your money and gives you an extra part and another gasket to fiddle with. In order to "increase the volume of the float chamber" without messing up the fuel level you would have to also lower the float chamber when you install the spacer.

Bottom line is, the only proper application of this part is in a picture as a conversation piece when discussing production problems of early Twin Cams and the gullibility of novice wannabe racers who may be itching to throw some money away for a trivial wow factor.