Seems weird but could well be the chain's too tight.
Whatever the book says, I try to do mine so that with me sitting on it there's a good half inch of slack still available for suspension movement over bumps. That translates in my case to more like an inch and three quarters when the bike's on the centre stand. But then I'm, er, large, and doing the adjustment usually gives me a pain in the side as I'm not so constructed that I can eat my toes etc etc.
Always far better to have a bit too much slack than too little. Why not ease it off a bit and see what happens?
As an aside, I don't find BSA pre-unit rear chain adjustment one of the more satisfying aspects of ownership, I have to say. There seems to a lot of difference in tension between no-load and full-load - more than on many other machines - which is presumably down to the relative positions of g/box sprocket and swinging arm pivot (although they don't look to be further apart than many another machine of the era).
