Zenith as returned from the so called specialist...
Had a look up close at the carburettor, found that it has suffered the usual body distortion.
Measuring the gap between the bottom of the float body to the main body on the emulsion tube side there is a 13 thou gap!
Measuring the the gap between the body and the float chamber there is a 6 thou gap! This means we can establish that it would have leaked fuel and ran like a three legged dog before going anywhere near the engine.
Picking up some tips from the forums the distortion looked quite minor compared with some I've seen, so I've decided to try and correct it by dressing down the float chamber top face to adjust out the distortion.
Stripped out all the jets etc to a bare casting.
I found highlighting the machined face with a marker pen would give a good guide as where the material was removed.
After some trial and error offering the chamber up to the body and measuring with a feeler gauge the gaps started to close up.
An initial trial with both gaskets fitted looked good but on one side there was still a 1 thou or so gap towards the top edge.
This was a combination of dressing the actual main body which had a high spot at the bottom and
years of undoing/tightening the two float chamber bolts. This had crushed the top edge of the body causing a small ring to foul the bolts, with this filled out it allowed movement closer to the main body and the gap closed up.
It's worth noting that with the amount of material removed from the top side of the float chamber that the emulsion tube gasket does not sit flush and may need to be trimmed down, but in this case it was an advantage because the float chamber top gasket did not but up to the edge of the right angle of the main body. When assembled it formed a perfect overlap with the other gasket...
The final thing I've noticed is that the two main jet access bungs have had fibre washers fitted that are too large. One has been cut down so they fit together, but it looks too close for comfort and just asking to leak fuel. Did they have fibre washers fitted originally or are they aluminium / copper ones?
After fully reassembling the float chamber and bolting it all back together and finally no gaps anymore, patting myself on the back I found this in the cardboard box for storing the carburettor....