Had exactly the same thing many moons ago when learning the black art of keeping old wrecks on the road. It would run - sort of, enough to get me to the pub, even if it did eight stroke a bit. The solution - I'd left out a paper washer that the jet block sits on in the carb body. I think the old one...

Hi David, FWIW I have CAMB1890 - and its in a bit of a bitsa too. It was in a scrambles outfit with an HT frame when I got it 40+ years ago. I swapped the rolling chassis for a '56 VH as one just happened to turn up at the time, & one day.... I must bring it back to life!
Mark