Hi Richard,
The Zenith can have some problems in a few areas, there is an o-ring in the centre that can get been pinched or damaged, the top can get warped and need to be sanded flat, the needle and seat may not be sealing properly and the floats can be adjusted incorrectly or even be damaged and not working correctly. All of these faults tend to cause flooding and running rich. Other problems can be caused if it wasn't cleaned properly when rebuilt and a jet is blocked.
Actually another thing I had problems with with the Zenith was that I bought a kit from Repco and the gasket between the body and the top plate was incorrect and it caused flooding when the drivers side was downhill.
Hope that helps a little 
TimJ.
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