Aprilia scooter Leonardo 150

This scooter runs perfectly but sometimes and becoming more often now, idles too high most times when I stop.
I thought it might be the autmatic chock and disconected the wires to it but the scooter still stars OK and runs OK, idles slightly slower until warm then back to idling too fast.
I have checked the carby and the butterfly always returns to the correct stop position.
Someone suggested, look at the vacumn lines.
Any ideas please
16,000 k's on the clock.
Cheers
PHIL in Australia