912iS not cranking

How old is the battery?

They are cheep and easy to replace.

Second thought is you may have a bad spot on the starter armature.

But it's unlikely that the starter would stop in that very same place twice so close in time.

I'm betting its a battery that would probably fail a load test.

Can't explain the prop turn reasoning, but like I said, it's cheep and easy to replace before you go deeper.

This is an interesting problem.

On the iS engine the starter relay is controlled by the Rotax Fusebox / ECU via the HIC-B connector.  While the information is not published, it's possible that certain conditions must be met before the Rotax magic boxes will engage the starter relay.  When you rotate the propeller in the normal direction, the starter is not turning because the starter sprag clutch slips freely in that direction.  So, I don’t think you are affecting the starter itself by turning the prop.  

The first question is, do you hear the starter relay click when you push the start button (even though the starter does not engage)?  If so, your problem is after the relay, which would only leave the starter itself, or connection problems, or low battery.  If the relay is not engaging, of course first check the relay and wiring, but I suspect the problem may be that no signal is being sent to the starter relay by the Rotax fusebox.  

If the 1/4 turn of the prop is really what is causing it to work (and not some other incidental action happening along side that), then you have to look at what is affected.  The only sensors I can think of that would be affected by a slight turn of the prop would be the crankshaft position sensors.  There is little information about how and if this could affect the start signal, but it would be interesting to download the fault logs via the BUDS software and see if there any clues.  Hopefully someone with more inside knowledge of the starter control on the iS engine will chime in. 

Edmond,

After posting my last response I read again your original post.  You state that both Lanes A & B were "on and illuminated".  Do you mean the lane fault lights were still on? That would indicate both lanes were still in fault (or boot up) mode.  I don't know for sure, but the engine might not crank with both lanes in fault. The lights should go out before cranking.   

Good idea, but the battery is almost new, and there is absolutely no indication of any load applied to the battery.

thank you for your response.  There is absolutely no sound or any indication of any load applied to the battery when I engage the starter (key driven, btw).

By both Lanes A+B illuminated, I mean the green lights associated with each lane are illuminated.  Master switch ON, Lanes A+B on, normal starting procedure, good battery voltage, reasonable amps for pre-start. There are no fault indicators on the lane switches, or on the Dynon display.  Everything appears to be normal, except for turning the key and nothing happening.