Well I decided to 'revisit' the problem symptoms again last night.
Rocker removed and the valves were regapped a pair at a time (a pair of feeler gauges, one per valve). They were miles off, far too large, at least 100% bigger than they should be. How did I do that!? So re-gapped the valves/rebalanced carb at ~1500rpm.
Now snappy off the throttle and 'almost' like a Swiss watch on tickover. So probably the excessive engine breaking was the exhaust valves were not opening anywhere near fully preventing the exhaust gases from getting out freely? I'll get it all back together and give it a run prob at the weekend. Need to rebuild the front brake first - seal kit on it's way.
Carb balance still shifts as the revs increase though by, say, 15cm from 1500rpm to 3000rpm.
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