Diagnosis

Now with the head removed I could look for the problem. I started by cleaning the head up in a paraffin bath, then I removed the four valves from no4 cylinder, the offending one. I also of course inspected the bores. There were no marks in the bores whatsoever, so I decided that the problem was more likely to lay in the head.

This is a picture from above showing the tappets and valves removed from no4.

A close-up of the valve seats. Intitially I thought this was where the problem lay. The top right valve seat has signs of burning, shown within the circled area, as does the top left but not as much, this would imply that the valves have not been seating properly. This would in effect mean that they have been staying slightly open, causing the loss of compression. Also allowing burning gasses to singe the seat. The cure for this would simply be to regrind the valves and seats. I did this and reassembled the engine. After our relocation move, I eventually got time to put the engine back in the car. When I ran it up it was still only firing on 3 cylinders.

Due to this I decided that there had to be an issue with the piston rings in No4 cylinder. I managed to get hold of another engine and have since installed that. It was a later model lpt engine, so should have had less mileage and wear. That now runs like a dream. I still have the other engine in the garage and will strip that down to find exactly what was wrong. It also allows me to rebuilt that to whatever spec I decide.