K3 starts well but runs poorly once its warmed up

[Colin Macdonald]

I've got a low mileage (7000mls) 1973 K3. It doesn't like the choke but once its started it runs really smoothly for the first 5-10 minutes. After its warmed up well it idles fast and is hesitant coming back onto the throttle. I've stripped and cleaned the carbs, set timing carefully, checked tappets. Haven't changed plugs or looked at needle heights in the carb bodies.
Anyone got any helpful clues?
Cheers, Colin

[750Ksabin]

Check for vacuum leaks around boots they maybe cracked also check the vacuum plug where the synchronizers would hook up.