I despaired last night and went back to the set of K-38s (which I think are indistinguishable from PZ-28. I checked the main jet which is marked '92'.
I've roughly set then up and it revs freely! I'm starting to think I'm an idiot!
I have a feeling I've been throwing too much fuel into the cylinders making them over rich, unable to run at speed, sooting everything up. I'm going to check the jet size on the Jikov and Dellorto carbs which I think was overpowering the 22hp Dnepr. So I've gone full circle!
Where did you get the slightly larger jets from? Or did you drill them?