Jeez, more I think of it more I think may be rings. If exhaust valves don’t fully open...air backs up in chamber and gets forced back and past piston? Piston rings (or valve seals?).

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280