Engine Idling High

Gene, you are not alone in not knowing about the black cap screw.  The idle adjustment procedure is in the 915 Installation Manual, and that’s where the black cap screw is described.  I’ve not seen this in the line or heavy maintenance manuals. Rotax seems dedicated to never repeating any procedure in more than one manual.  The only problem with this is that unless you built your own aircraft, most owners have never read the Installation Manual. I recommend reading, or at least looking through, the Installation Manual because it fills in some of the information gaps.  I think of the three manuals as chapters of a single document.   

I've adjusted many Rotax fuel injection engines where Jeff is describing where the black cap is. The idle rpm's on these engines were too low. The black cap covers a screw on top of the air intake where you see the cable attach. Plus sometimes this screw was so hard to turn it would have snapped off. I heated it up some and then it turned easier. 

Hi all

Here is what Jeff is pointing out...from the installation manual

Cheers

Hi Roger,
I believe the reason the screw with the black cap should not be turned is that it’s more than just the minimum idle setting. It is set at the factory as the baseline position for the calibration of the throttle position sensor. This is why the second screw (with the spring) is provided as an adjustment point.  However….

I’m wondering if you are referring to adjusting the black capped on early model 912iS engines, some of which don’t have the secondary adjustment screw with a spring  (like my aircraft, see photo below).  Still, I don’t think you are supposed to adjust it. On my aircraft I use the idle stop on the aircraft throttle lever to adjust the idle up, the black capped screw is set low enough it does not interfere.

Hi Jeff,

"I’m wondering if you are referring to adjusting the black capped on early model 912iS engines,"

I was.