[dr bob]

That's "ported vacuum", and increases as velocity and mass through the throttle area increase. Same as higher load on the engine. If you T your gauge into that line, you should see more vacuum at higher RPM's and more throttle, within some limits past half throttle.

And I'm sorry I wasn't careful enough to notice that you are in Scotland rather than the US.

I should also suggest that you start off with common maintenance items if they aren't current. Plugs, ignition wires, distributor caps and rotors top the list.

Also verify that the belt in the distributor is intact if you have the twin-distributor-cap motor. Just pop the forward cap and see if you can turn the rotor any by hand, indicating a failed belt. A failed drive belt will cause four-maybe-five cylinder operation, very rough by any standard. Easy and inexpensive to fix, in the big picture.