Typically an exhaust valve problem, burnt valves are usually the result of hot combustion gasses escaping between the valve and valve seat when they don’t seal properly. Adequate sealing between the two also allows combustion heat to be conducted from the valve and into the seat, preventing the valve from overheating.
Detonation and pre-ignition can also cause elevated combustion temperatures and lead to burnt valves, so make sure you are running the recommended fuel grade. Carbon deposits can also prevent the valve from properly seating and lead to a burnt valve condition. If the valve no longer closes completely, the valve and seat will burn.
Another cause of burnt valves is valve seat recession. If the valve no longer closes completely, the valve and seat will burn. Carbon deposits can also prevent the valve from properly seating and lead to a burnt valve condition.
Doing regular leak-down checks may help prevent this from occurring as it will provide clues as to how well the valves are sealing. If you are getting low readings, it indicates that cylinder head work (lapping valves) may need to happen.
Other possible causes include:
- Overloading the propeller
- Improper purging of the oil system (Flat lifters), need to examine the lifters and valvetrain for signs of hammering and replace all worn/damaged components.
- If the cause is detonation or pre-ignition, you need to check the piston, cylinders, and other components within the combustion chamber.
- Loose valve guide
- Cocked or weak valve springs
- Lean air/fuel mixture (check for induction leaks) ie. cracked carburetor sockets.
References:
Service Instruction Rotax released regarding suitable operating fluids for ROTAX® Engine Type 916 i (Series), 915 i (Series), 912 i (Series), 912, and 914 (Series) - https://legacy.rotaxowner.com/si_tb_info/serviceinfo/si-912-016-r13.pdf
Video on how to perform a differential compression test (leak-down test): - https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/181-diff
Video on how to prevent detonation/pre-ignition - https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/201-detonation119
Video on purging the lubrication system: https://www.rotax-owner.com/en/videos-topmenu/expanded-video-instructions/33-exp-si-912-01813
