I have posted a few photos, lemme know what you think!First......welcome to the forum imtaylors
This is an excellent suggestion from G because at a stroke it will confirm if your problem is due to the carburettors or the cable/s & throttle assembly . You can potentially eliminate unnecessary checking and concentrate your attention where the problem lies
If the throttle shaft return springs haven't been reassembled on the correct carb (they have different strengths on BS38s and may have on your BS34s ) or haven't been assembled on the shafts with the correct number of pre-turns then that could cause a slow/ weak throttle return.
Your carbs are presumably BS34 . it might be necessary to add some additional spring/s to the throttle shafts to compensate . Your original return springs will have lost a lot of their spring strength over the years and aftermarket replacement springs may give little improvement.
I attached a strong coil spring to my throttle shafts and the throttle shut down is now nice and snappy
Hopefully these photos do some justice. And hopefully show you clearly what the issue may be.