As noted above, there have been issues with Zenith carbies (and copies).
Most of these relate to the seal between the top part of the carburettor and the main body. There are two issues. First, and most common, is that the top section is not flat on its mating surface. This needs to be flat to properly seal several passages going between the top and bottom. If you take the top off, and remove the emulsion block, float and needle valve, the top can be flattened using a piece of plate glass and fine wet and dry. Take care in reassembling that the screws holding the emulsion block are not burred - there is not much space for their heads, and a burr on them can undo the work you have just done.
The other potential issue is that this design depends on the O-ring in the throat to stop direct suction of fuel from the bowl. Any damage to the O-ring or the sealing surfaces can result in a leak as can a hardened O-ring. It does not take much, as there is a high vacuum here at idle.
However, as you found your distributor loose, I suspect this may be a lot of your problems. If you can get it idling and rotate the distributor for the best running, and clamp it at that, it will be pretty close.
Another potential issue is air leaks anywhere into the induction system - most likely is the gasket below the carburettor, or the vacuum advance line, or the PCV system (if fitted), but manifold to head is also a possibility.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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