[jpnovak]

So... If you are having trouble running leaner than 12.5 you need to add more timing. A rich mixture is slower to burn - up to a point.

General rule of tuning is that to run more lean, you need to add more timing. This rule especially holds at light load and cruise. This is why you see Motronic (3.2 and 964) cars with lots of light load timing. In some cases, higher than 40 BTDC.

Increase the timing at light load and see if that helps you increase the AFR to run more lean.

And yes, Always give the engine what it wants... In some ways you tune by feel/sound.