Thanks for your advice bigted.
When I dismantled the mower I had both the carby and muffler off, so I could see the piston and rings OK. They looked fine. So I don't think the problem is in the cylinder. As it was going just before I pulled it down, the problem is more likely the carby setup, as the spark seems OK.
I put two brass spacer washers into the G4 carby. This may have altered the mixture at idle, which is where the throttle lever is at cold start. I think I need to remove the carby and then remove the washers to take it back to original setting and see if I can get it to start.
Styx
