Chesty, it's under the throttle body - right hand side (or thereabouts, it's a few years since I've had an ERST) large-ish knurled screw with a spring (don't let this drop when you unscrew it!)

Best to clean it with petrol or similar as it will be coked up. Then when you screw it back in all the way, unscrew approx 2 full turns (remember we're taling YEARS ago since I had one!) but obviously check the idle speed / stability.

It adjusts the air mix at idle IIRC, not the actual idle speed screw.

EDIT: would have beaten Rick if I didn't type so much