MSefi.com read only forum (Jan.15,2018)

So during cruise (2000-2500 RPM), if I reduce my timing and don't change fueling, the 02 sensor reports a leaner burn. Why is this?

If I pull it back 3 degrees. Example: I change it from 20 to 17 deg BTDC, it will change the AFR from 16.0 to 17.0.

I'm just trying to understand the basics of this concept. Anybody think about this before? If the sensor reports a leaner burn, does that mean I've achieved a more complete combustion? I would think that a more complete combustion is desirable....as long as the piston receives peak pressure at the optimum time.

If the sensor is reading leaner, it means there is more oxygen left over. If the amount of fuel hasn't changed, then the only answer is that LESS fuel has burned.

Hmmm, I'm gonna have to stew on that.

I believe that less timing allows less time for the fuel to burn. IMO, More advance would allow time for complete combustion at least up to the point of detonation.