[Gunk]

I've just replaced the complete idle control cable assembly on my F3 and Ian is right if the spring is missing, engine vibration will cause the screw to undo and the idle to drop, so firstly I'd remove the fuel tank and check.

The bad news is that I don't think the screw Spring is available separately and the complete idle cable is difficult to source, I managed to get one via David Silver Spares here in the UK for £20

However you might get lucky and a screw out of a biro may do the trick.

 

[Gunk]

small piece of rubber tube will do the same job, all it needs to is to hold the screw in place