The O-ring between the top and lower parts of the Zenith is probably leaking. Try tightening the four screws holding the top on. Don't overtighten. Last time this happened to me a quarter turn on the two screws nearest to the choke fixed it.
If that does not work, carefully remove the top, taking care to remember how the linkage from the choke and acceleration pump to the throttle etc is assembled. (no need to remove carby from vehicle, but don't drop any bits!)
Examine the mating surfaces that seal on the O-ring and make sure there is no foreign matter and that the O-ring is undamaged. Make sure the screws holding the emulsion block are tight not hitting the bottom part of the carby and stopping load on the O-ring. If nothing found, remove the float, needle valve, emulsion block and gasket. Check the top cover mating surface is flat. If necessary, lap it flat with wet and dry on a sheet of plate glass. 
Carefully check all jets are tight, and that there is no foreign material under the needle valve. Reassemble, taking care to tighten the top cover screws evenly and gradually to give the best chance of sealing the O-ring. A smear of grease may help it seat more readily.
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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