That's easy to say but a lot harder to do. The stock sensor output toggles between a low voltage and high voltage as the A/F ratio passes through 14.7:1, and the computer is programmed to interpret this switching as representing unity.. or the A/F ratio it is programmed to work towards. What you need is a sensor that toggles at 15:1 or so, I don't know if these exist in a simple narrowband sensor form, and if not you would probably have to start with a wideband sensor that has some custom electronics to simulate a narrow band sensor that switches at your desired ratio.

Innovate Motorsports has such a device, I actually have the sensors kit below but have not installed it yet. This unit has programmable analog outputs that simulate a narrowband sensor, and it appears you can adjust the A/F ratio it switches at. Have a look...
LC-1 Lambda Cable with 02 Sensor : Wideband Controller Cable for Dyno, ECU, Data Acquisition, or Gauge Applications