Well, finally got tired with trying to get the twin Zeniths set right to get this thing to run better, after messing with varying combinations of them and a few disappointing test runs I finally gave up spent some money bought a brand new Stromberg 81! Made up a new manifold to fit, it ran very cold and rough with lots of condensation, most likely cos of the small venturis. So I added a water heated bit underneath and I'm pleased to say it now runs very well now, revs up clean and smartish. Need to get it out for another run, but I think it'll be pretty good out of the box. I might even re-make the manifold in ally sometime, or maybe wait till I get the motor rebuilt and make a second one for a pair of Stromberg 81s on it!