when revs are dying down almost back to idle there is a knock/pop (metal to metal sound) coming from the head. Sounds as if the piston is hitting the head?! taken it all apart. Piston/Rings seem fine. No play in big end bearing. Slight play with small end bearing when you move it side to side but thats normal so im told. checled carb/exhaust all seem ok. When you rev, its revs up fine. Poping/knocking only hapens when revs dying down. i run 75ml castrol tts per 5 litres unleaded. Help.

Problem solved. checked timing, cleaned carb, new plug, new (thick) base gasket, new head gasket - now runs like a dream!!

How thick was the new base gasket? My 240 makes this noise with normal unleaded petrol but seems better with Super unleaded, I'm not sure it's completely gone though.