PA39
The lines up to the cylinder from the FCU contain no pressure once the mixture is at ICO.
OK, this is my observation of why it is the TCM and Lyc are different. In my opinion and observation a TCM will not pump much fuel at cranking RPM which is very very low. The overly rich situation eventually (and quickly) finds the right F/A ratio and lights off. The low RPM and thus low extra fuel delivery means no excess post-priming fuel will dribble in.
The Lycoming which is now primed up and overly rich, waiting for the F/A ratio to get optimal will start pumping fuel according to mass airflow immediately (and at the FR position this will be very rich….and will be too rich to start when combined with the primed amount.
Two different systems, two different techniques.
