Engine Idling High

Does it only idle fast after initial start when the engine is still cold?

No I noticed it yesterday as I has an extended wait for takeoff due to landing traffic.

I know most fuel injected cars will idle fast for a short time after start while they warm a little, but not sure if Rotax uses similar logic in its ECU.

The Rotax FADEC does not have any means of controlling the throttle body like a car engine does. It can control the injector pulse width, and cold air is more dense than warm air so that can affect idle speed. Regardless of the reason I believe your only remedy is to adjust the idle stop position. The 915 has two idle stop screws, one with a spring under it and one with a black cap on it. The screw with the black cap must NOT be adjusted, and its factory set position represents the lowest idle limit. Typically the screw with the spring is set to touch first, and you can adjust that screw, keeping in mind the lower idle limit is when both screws contact at the same time.

It’s also best practice to have an adjustable stop on the throttle lever in the cockpit and most aircraft do. Ideally this is set to touch just before or at the same time as the stop on the throttle body.  This stops the pilot from putting stress on the cable and brackets if they pull the lever hard to idle. Often, when the idle is high it’s this throttle lever stop that’s hitting and must be adjusted.  Regardless of what adjustment is made, make sure the throttle stop in the cockpit (if you have one) touches just before or at the same time as the (adjustable) stop on the throttle body.

Jeff:

Great suggestion. I will take a look at both stops.  Never knew about the black cap rule. 👍👍