OK I've examined the vacuum pump on my Holley mainly to check if the diaphragm is OK (it is). I also found one of the mounting screws which wasn't tight. Anyway, it seemed that the little ball bearing was jamming in its hole preventing me from moving the diaphragm by sucking on the housing. I thought who put that there? I removed the ball bearing and re-assembled everything. The car seems to run fine, but of course this ball bearing is the check ball which Holley put there for a reason. I presume I should put it back in there, but what exactly does it do? Cheers, Mark.

The ball should definitely go back there.
It keeps the secondary blades from snapping open (or shut?) too suddenly when there's a change in the vac signal.
I can't remember off the top of my head whether it effects the sec closing or opening, because my carb isn't on the table here in front of me and I can't find a diagram. But I know the ball should be in there.

It slows the OPENING of the secondaries (it becomes a very fine vacuum restriction when sucked tight against it's seat by venturi vacuum fed to it via a passage built into the main body of the carb) but unseats and allows them to shut immediately when you get back off the gas.

BTW- Make SURE the little round rubber/cork "donut" gasket that seals the vacuum chamber assy. to the main body of the carb is PRESENT and IN GOOD SHAPE before you put the vacuum chamber back on the side of the carb. If that little gasket is missing/leaking the secondaries will never open.

Yes the cork gasket seemed fine to me. Cheers. All I have to do is go and get the mixture set right - she get really crappy mileage, like 10 mpg!! Don't get me wrong, I didn't buy Morticia for fuel economy.

Mark.