Thanks Steve. I checked the duty cycle with key on, engine off. It was 85% (it is a California car). The problem is only with the engine working, that is when the duty cycle is 100%.
I will try the EHA method you mentioned. I have a good precision ammeter and I will connect it to the EHA and monitor the current as I adjust the mixture screw. As far as I understood from your article I should adjust it to get an EHA current of ZERO mA. I assume that this is done with the engine at operating temperature, correct me if I am wrong.