I've been researching the FAST system for a project I'm working on. Seems that some of the controls in the setup software will allow you to go to negative settings. This apparently is not very well documented in the manual.

At least one person I read about had a rich idle problem that actually caused flooding and stalls. The control for idle mixture was set to "0" and it was still rich. By accident he discovered that the control would actually go below "0" and that cured the problem.