You can't "lean it out". The 02 sensor and electronics will maintain an average stoichiometric mixture assuming everything is in proper working order.

The initial mechanical mixture adjustment as determined by testing the "duty cycle" just establishes a basic mechanical mixture setting near stoichiometric so the lamda system,s control authority is not pushed to the limit.

M103s tend to have high emissions and you must manage your test. I've discussed this in many, many posts.

Duke