Thanks for your response here. The distributor was new following a recent full re-build. I've had a look inside and it all looks clean and intact, weights and springs all in place, points gap correct. The bike's been running ok (apart from the high revs).
The Monobloc carb has been removed, dismantled completely (again), thoroughly cleaned with carb cleaner and compressed air, not forgetting the very small holes that are just about visible to me. I have now added a smaller 'inner' spring along with the original throttle slide return spring which gives a satisfactory throttle return feel. The throttle slide does show some signs of marking only very slight in my opinion but the slide returns very positively now.
Started up and set the mixture and throttle stop settings and all seemed well until it warms up and the old problem is still there. The revs do not always return to normal straight away after a twist of the throttle. It takes 2-3 seconds from throttling back to settle back down but as the engine gets warmer this time delay gets worse and in some instances would not settle back at all. The odd thing is that when it's revving high, the throttle stop screw does reduce the revs ok that is, until the next time it's started when it wont tick over !
There are no air leaks at the manifold
All carb components are very clean
The float level has been checked and spot on to the dot on the float chamber cover level.
There is good spring return on the throttle slide.
There is no air filter
The manifold nuts are not overtight.
Using a colourtune, the spark is a decent blue colour at tick over. Not when revving though (goes yellow)
The slide does have some minor marks but moves freely in the body.
With battery cover off and engine revving high, the throttle slide can be seen minutely 'vibrating' within the chamber. Is this normal ? It just takes a touch with a plastic tool to stop the vibrating and then the engine immediately dies.
The engine runs ok when on test rides - no misfires etc
The carb was new in 2016 - only done 300 miles since.
I'm now thinking that maybe there is a problem with the carb body which may be causing this issue. Does anyone have any other thoughts or suggestions ?