Aux Pump

I think the best thing to do is to follow the Rotax use manuals for things engine related, not the planes POH. I always use both pumps for pattern work and for take off and landing, and turn aux. for cruise. Pressure should always be between 2.9 and 3.2 bars, if I am not mistaken, regardless if both or only one fuel pumps is active, but having both on critical phases of flight brings welcome redundancy.

Albert,
Read the Rotax 912iS operating manual, section 3.4.  They are very clear to only use one pump when starting, even stating that using both pumps during starting may cause a bad starting behavior.  Just as important is to set the throttle to between 55% and 65%.  This should be done using the throttle position readout on your EFIS, since the physical throttle position is not a good indicator.  It’s typically only about 2cm forward.  

With a fuel injected engine a higher throttle position delivers a longer fuel pulse, so it’s similar to using a choke on a carbureted engine - it gives you a richer mixture while starting. Using these settings, you will need to reduce the throttle immediately on starting to get the RPM down to 2100.  

55 to 65% throttle may be a bit high for most ops. See the more detailed discussion on page 3-9 and the diagram that shows how it is related to temperature.

I saw the “bad starting behavior” comment in the manual. I think that was for a carb engine. However, when we hit the start button several flights ago with aux pump off, the engine didn’t start. Now that’s ‘bad behavior’. So the last 4 flights, we started with aux pump on….& each time the engine started right away. Note:  this machine (N191TL) only has one aux pump switch. 

Thanks.  I’ve seen this chart, but didn’t connect dots. We’ve been starting with the throttle at idle. So we will set the throttle higher & pull back right away to see if that yields consistent starts.