Have to conclude that I haven't got far on this. I was sure this was some sort of hydrolock.
In summary: compression gauge measures 120 psi, able to pull approx 8 inches only, pretty jerky motion though. Without spark plug in it easy to pull starter cord.
Assumed it was hydrolock so pulled starter cord many times without plug to try and clear oily residue, then tried to clear it with a liittle petrol and oil in spark plug hole, thought there was some improvement. Left it for a few days, starter cord was again difficult to pull. Took muffler off and tried to run compressed air into cylinder bore, again I thought there was some improvement but after a few hours it is again difficult to pull.
Maybe I've misdiagnosed this and it's a starter problem but why is it easy to pull without the spark plug inserted ?
