Best way to tell how much effect is to idle the car and move them around
and then oil them with some fairly heavy oil so the oil gets sucked
into the space. See how much it changes the idle.

Ideally you shouldn't be able to move them perceptibly, but little in
this world is ideal. What it does is to make your idle have to be set
rich to run right (because of additional air requiring additional fuel),
so when speed is up and throttles are open, it is quite a bit more too
rich because the throttle shaft leakage is a much smaller proportion of
the total amount of air admitted.

(Pardon the bad syntax)
--
George Downs Bartlesville, Oklahoma