Can any body tell what is this screw for?

It is located on the idle control valve

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Hi 260,
It's the factory set bypass. When the valve is not actuated, that bypass defines your idle speed. Under the seal is a small allen-screw. Don't waste your time though, cause there's always a reason why you have an idle-problem, and it wont be the setting of that screw.
Clean out the valve with some carb-cleaner, then WD-40 treatment and ensure all other components work OK.
All the best,
Dokta

Many thanks Dokta tha twas my guess,

I think that with ageeing, the stop of the shutter may be a little bit worn out and would need to be adjusted (indeed with all the rest in order.

Do you know if turning the screw clockwise it shuts or opens the by-pass?

I bought a used Idle control valve & noticed the seal was gone & there is no screw. Is the valve useless?

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Hi 260!
What usually fails is the spring (tired) in the valve and it's shaft (seizure). I don't recall the direction of turning.
All the best,
Dokta

Many thanks Dokta,

The idea is to compensate a little the ageeing of the stop of the shutter which is activated by the spring (or to gain a litle more RPS at idle).

Hi 260,
You can't alter the idle on a functioning system with that screw!
It defines the idle when the valve is NOT actuated. e.g. electr. failure. It is the factory setting for emergency run.
All the best,
Dokta