Unplug the cable from the throttle switch, and jumper the pins/sockets that correspond to switch pins #1 & 2. This will simulate a closed throttle switch of 0 ohms. Any difference in the idle, or in response to changes to the idle mixture?

The duty cycle will oscillate between 45% and 55% when:
1) the engine is at operating temperature
2) the O2 sensor is above 600 deg.F and supplying a valid signal
3) the EHA is properly calibrated to provide a lower chamber pressure that is .4 bar below system pressure at a current of 10mA
4) the mixture screw has been set to provide a stochiometric mixture
5) the ECU is working properly