Thanks for your replies I got the info I needed , my German XBR was a 27 bhp detuned model
they fit a 168 main jet and 14 T front sprocket and a restricted rubber inlet tube.
Mine had the full power inlet tube fitted but not the correct main jet and 15 T sprocket
All fixed now hopefully .
13k miles and as new condition for £1500 with wire wheels too ,happy now.