Hi Roger, just a note regarding

'however I suspect it is enough to slow down the reaction of the MAP sensor to the Vacuum drop under accelleration particularly when the engine is cold, which in theory would be enough for it to go weak for the couple of miliseconds'

A split in the vac pipe to the Map sensor will give a complete vac loss when opened (depending on the size of the split of course) which will force a rich mixture not weak.

Reagrds
Sean