Thanks for the replies gents.
Engine is completely standard with standard gearing. Plugs are proper tan colour when doing a 'run and shut off' test, although it does run slightly rich at tickover. Sorry to sound a bit thick, but what's the correct procedure for checking the ignition timing. I was under the impression that once the timing had been set, it wasn't likely to slip (I had help on this). Also, I understand the '61 GF had auto advance/retard, so how would I end up with retarded ignition?
For the record, when I say I wind her up, I mean taking a run at hills to mitigate the issue. Obviously, when it starts to slow, I bang it down a gear. Unfortunately, on the worst hill across the Derbyshire Dales, I'm often struggling in 1st and barely make it to the top. All other times the bike runs like a dream??
This is my first restoration on a British bike so I'm still coming to terms with its eccentricities
Cheers