It's not clear to me what you are measuring. If you are measuring the EHA duty cycle according to:

www.landiss.com/mixture.htm

the throttle valve stop and switch have nothing to do with it. The adjustable throttle stop is on the opposite side on the throttle valve shaft from the switch as I recall, and has a yellow thread lock compound. Do not confuse the throttle valve stop with the air flow meter adjustment.

When the ECU receives the idle signal it activates the idle control system and the throttle stop should be adjusted only if the idle control system duty cycle is not in the proper range. This is a completely different measurement and system from the EHA.

If you ARE measuring the EHA duty cycle there is something wrong with your system or you are not measuring the duty cycle properly. The Landiss web page should help you to be sure that you are measuring the EHA duty cycle properly, and note that there are differences between CA and 49-state cars.

A VOM should read about 480 mV on the O2 sensor signal lead. With a scope it should cycle between about 0.2 and 0.8 volt. You can spy on the O2 sensor output by pulling the connector under the passenger carpet enough to attach the scope probe. The companion two lead connector is the heater and it should show continuity on the O2 sensor side if disconnected and system voltage on the chassis harness side.

Duke