Thanks everybody for your input. Based on your notes and my own experience my theory and it is only theory, is that there are two distinct symptoms arising from the same fault. The fault being the lost motion or backlash in the coupling. The symptoms being:-
1/ at low revs when running light the constant change from drive to overrun causes the coupling to slap to and fro in its own backlash and -
2/ At high power transmission combined with minute loss of traction the coupling whips back to the overrun position causing the bang and immediately takes up the backlash again when the bang can be repeated several times till driver reaction or road surface change breaks the cycle.
The noise is probably generated by the sudden take up of slack in belt or, in my case chain, final drive, which is why so many seem to think it is the belt jumping over the sprocket teeth and also why the noise is less when the drive belt is run very tight.
Mention is made, above, of " soft return " which means the unit works in both directions and photos published here seem to confirm this. I am not sure if this makes my conclusions more or less likely but it seems to indicate that some attention has been given to eliminating any banging when the backlash is suddenly reversed.
My solution is to eliminate the backlash with moulded rubbers which completely fill the cavities in both inner and outer elements of the coupling and which, therefore, do not roll back and forth. This will be a much cheaper fix than the replacement of the coupling with another design altogether. It may be some months before I can devote time to this but as soon as I have conducted a good trial I shall publish my findings here and elsewhere - successful or otherwise!
P.S, I seem now to get no response from the factory or Centa when I mail them. Like so many, they probably think I am mad!
