It sounds like you are using the correct duty cycle function of the volt meter.

Usually an error code will be a solid number, ie 20, 30, 50 ect. A solid 20% reading indicates a "wide open throttle contact has incorrect polarity or is defective" But it also states this reading only 20% when decelerating and the micro switch is activated.

Since you are seeing some fluctuation, I don't think this is your problem. Any other possible leaks in the system? Leak from the EHA or the O ring on the fuel distributor? At 19 - 31% something is causing the O2 sensor and computer to see a rich signal.

You could:

1. Keeping leaning it out and see what happens.
2. Sometime the O2 gets fouled. Why not remove it and look at it and put a torch on it to get it rosie red to burn/clean it off, or
3. Monitor its signal to see what voltage it is sending the computer.
4. Check the EHA current to see if it is in agreement with the duty cycle.

Just some thoughts.

Tiinker