Grumpy,
Sorry missed the last two numbers 790531 so built in 1979.
I moved the spring to the notch in the leaver and the tension profile is much better see attached picture
Ran the engine, runs steady and revs up nicely. I need to check the high idle speed with a tacho to make sure it is set up correctly.
In th e mean time I'll look for the rack and pinion style control box
Cheers
Mick