Rotax 912

Fixed.

With the exception of a wee bit of retard if you have an engine with (connected) "Soft Start".

(From Bill Martin "The engine starts with the timing at TDC and after a few seconds changes to the standard BTDC setting.")

Optimum engine RPM is around 5200 (range 4800-5600 ??).

Few, if any standard aircraft engines have variable timing (past starting aids). Car/motorcycle engines do and this may be kept or not, when converted for aircraft use. 

Just like your lawn mower, aircraft engines are designed to operate within a small RPM band. Operating outside the recommended engine speeds is likely to lead to a shorter engine life.

Having a narrow engine speed range allows the designer to strive for minimum weight/best power : weight, optimum volumetric efficiency, cooling and long in-service life. 

Without getting into too much,

If you have the green striped labeled ignition modules they start with 4 degrees BTDC and after the engine starts goes immediately to 26 degrees BTDC.

If you have the yellow striped "soft start" system then the engine starts with 3 degrees ATDC and then after start it stays there for 4-5 seconds and switches to 26 degrees BTDC.

Check out these two free video courtesy of Lockwood Aviation that helps explain things!

https://www.rotax-owner.com/en/videos-topmenu/free-videos/695-Rotax-lockwood-ignition-troubleshooting

https://www.rotax-owner.com/en/videos-topmenu/free-videos/708-rotax-lockwood-easy-start