Manwell
In case of the Tomahawk failed valve did you find the engine ran quite rough right after start up but smoothed out as the engine warmed up ?
i ask because an engineer diagnosed a failing valve in a flight school airplane I was flying. In that case the cylinder was repaired before the valve failed but in his opinion he thought valve failure was imminent. After that I always look out for this symptom particularly in the small Lycomings which are.known for exhaust valve guide problems