Idle speed too fast

[simmitc]

As it says, the idle speed on this one is just too fast. Checked so far:

Throttle cable - released completely, so no lack of slack causing a problem.
Choke cable - released completely, checked that jet springs back to normal running position.
Fast idle screw and throttle stop screw both removed completely, so they are not holding it open.
Jet centred.
Piston rises and falls smoothly.
Mixture fine using the piston lifter test.
Ignition timing "perfect" - retarded very slightly from when pinking occurs.

The engine starts perfectly, runs smoothly, and idles smoothly; just too fast.

Ideas welcome! Thanks.

[aupickup]

does the butterfly return properly or is it stuck a bit open

[bmcecosse]

Or there is an air leak....