Engine not slowing

[BlackDogZulu]

Recent work (20k mile engine): new exhaust headers, new inlet rubbers, cyl head and piston decoked, valves reground. Only work on the carb was to take off the float bowls to check float heights, which were correct.

The bike now runs fine, and seems to have a bit more power and be a bit smoother than before (one of the inlet valves was showing signs of blow-by). But there are three things not as good as before:

1. Starting takes longer on the button - it was almost instant before, now it takes 3-4 seconds of turning

2. Bike will occasionally stall during slow running, e.g. in traffic, when the throttle is snapped on and off

3. Revs take longer to return to idle when throttle snapped shut. Before, with a closed throttle the revs went back to idle immediately, now they die down slowly over about 5 seconds. This is a problem, as it means there isn't as much engine braking.

The throttle quadrant is snapping back onto the stop, so it isn't a sticky cable. It all says 'weak mixture' to me, but I can't think why. Anyone got an idea?

(Forgot to add: plug reading is mid-brown, perhaps slightly darker than ideal.)