The vacuum advance was still connected but this seemed to be doing something peculiar. On idle revs, when the throttle was blipped the ignition retarded by 12 deg.
I checked this against the other distributor when not installed to see how the vaccum unit moved. This distributor's max advance stop had been welded but was otherwise the same. The vacuum pipe was OK but I took it off and cleared it anyway. Once the revs picked up to 1000rpm the advance seemed correct, so maybe it just needs some different programming.
I tried a couple of curves and took it out for a test drive. Well it had to be done as the sun was shining and 24degC!
I stopped in a layby to reprogram bringing the advance in sooner to the curve as above. this was to try to get more power at the top end as the engine seemed to flat line at 3500rpm. I suspect it is the pair of carbs that need refurbishing. Anyway the car still cruises at 70mph in overdrive so will do for next weekend.
Maybe have a go tomorrow, if I can find time between packing to go to France very early Friday.
By the way, in case you are wondering about dwell the standard Jowett distributor should be set to 66% which is a (big) compromise. The curve above saves power and burning the coil out at low revs and gives the necessary energy at high revs. I will however, try the engine sometime with the standard Jowett set up and see if I can detect any difference. Really it needs a rolling road and dynamometer for accuracy.