912 IS engine starts astero and vibrates a lot, then it works perfect

Thanks for the clarification.  I’ll experiment the next time I fly.  

I have been starting mine about 2-3 cm forward, which looks like about 40% open.  It starts well, but does have that 1/2 second of abrupt vibration. I went out today and tried 50% and 70% throttle positions, both which resulted in needing to reduce throttle down to 2000 RPM after starting.  This did not seem to change things much, but possibly the vibration was a bit shorter.  The Rotax manual also says to hold the start button until 1500 RPM, which is pretty hard to do precisely, but I did try holding a bit longer after the engine fired.  Again, not much difference.  Its my understanding this phenomenon has to do with the fact that ignition timing is being controlled by the crankshaft position sensor, and the prop inertia affects that timing at low RPM.  I believe that this is the reason the zero degree dog hub is used in the gearbox on the injected engines.  The 20 or 30 degree hubs would cause havoc with the crank position sensor at startup.