Hello! I own a part in a 1999 model Ikarus C42 with a Rotax 912UL, new in 2007, approx 600 hrs. When we turn the propeller for «burping», every 4th stroke it emits a wheezing sound that appears to come from the muffler. It makes me worried, because I fear something is wrong with the timing of the exhaust valve – that it opens either before the combustion cycle is finished or too late when the piston already has moved towards the cylinder head and the pressure is mounting. We have tested the compression of each cylinder, and it is in the lower normal (Don’t remember exact values.) How serious is this? Grateful for any suggestions and recommendations.

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