carby volume screw has no effect

Hi all. I've got a 1979 SIII 4cyl petrol. I've just had my Zenith 36IV carby overhauled by a specialist. When I went to adjust it I found the volume screw has no effect (well that's not quite true; if i screw it right in or right out it will stall) but it's impossible to tune it.

I think it was like that before I had the carby overhauled and I assumed it was a carby problem. I took the landy to the guy who overhauled it and he's not sure what's going on - he thought it might be something wrong with the distributor setup.

Does anyone have any bright ideas about what the problem could be?

Is there an air leak in the intack manifold, or a the base of the carb? Had that problem with my weber carb and caused it to be impossible to tune.

This could be one of the common problems with a Zenith.

When you say it is impossible to tune, what exactly do you mean? How are you measuring the effect of the volume screw?

jp