Restoring a Top Hat

[Parazine]

Thanks Ruairidh, I've actually migrated on to a 24T-2. The symptoms are exactly the same though, with each carb type, the tickover speed is unstable and the air screw (or slow running adjustment for the 22FZ) has no effect at all. I tried a second 24-T2 to no avail and the same carbs on a different engine run fine.    I thought maybe the magneto was causing the problem so I put the spare on (again, proven on a different engine) but no change in symptoms. All the gaskets, flanges etc have been filed flat and sit snugly, no sign of any air leaks.

What may be happening is that, because the engine is tight, it takes quite a bit of energy to turn it over at tickover, hence the throttle stop screw is wound in, cocking the throttle open by a fair amount. Because of this, there is little depression on the slow running circuit (actually a different, small carb within a carb) and the engine is running on the main jet at tickover (or probably,  the progression jet in the case of the 24T-2). In this case, I would expect things to return to normal when the engine loosens up, although at present, it is still very stiff..... 
220 miles up now, perhaps it will loosen up by 500 miles. 

Top speed has gone up to 35mph with occasional, lightly loaded, brief bursts to 40.