idle speed too high sometimes

[lance280zx]

my 1981 280zx 5 spd manual non turbo car normally idles at 900 rpm or so. but sometime the idle will go up to 1500 or 2000 rpm.
I do not see anything obvious restricting the throttle linkage from returning all the way back to idle. there is some sort of leak at the idle speed screw. if i cover the hole that the screw is in with my finger the hissing sound stops. is there supposed to be a leakage here? What other devices should I investigate to lower the idle speed?

[icice9]

the cause of your high idle is the vacuumme leak. fix that and check your TPS as well.