What was it?
The timing at TDC would have been the issue. The vacuum advance really only affects fuel economy at cruise. Would not affect anything at idle or while under load.Hard to be definitive. Primary suspect is duff vac advance unit. I have loaned a Lucas rotor arm and I also re-routed the HT leads. Oh and the ignition timing was at TDC, so advanced it up to 5 degrees. Apart from that I would say it's a sweet engine. Hopefully the vac unit will sort it.
The timing at TDC would have been the issue. The vacuum advance really only affects fuel economy at cruise. Would not affect anything at idle or while under load.
I think you may be confusing manifold vacuum to that used for the vacuum advance. When coming off the gas they would be no vacuum to the distributor. Neither would there be on sudden acceleration. Power likes the timing retarded and therefore advancing the timing would make acceleration worse. Sudden acceleration is catered for by the TPS and the ECU adds a spurt of fuel to stop the stumbles.But it does help to stop the engine stalling when you come off the gas for say traffic lights and roundabouts. Also, there is a momentary increase in vacuum when you open the throttle quickly before the manifold then pressurises under WOT. Non functioning vac advance will cause the engine to momentarily bog down which is exactly what it was doing.
Mid range and under load it was fine, actually quite a sweet running engine that one.