Hi again Mark, gave the decompressor a quick clean, put it back on, same result ie starter jerks. When the decompressor was off and the hole uncovered, the starter did not jerk.
I tried putting a spark plug into the decompressor hole and blocked off the hose that was connected to it with some cleaning towel, the starter jerked again and mower did not start. The muffler is a little rusty around the mouth but is up against the engine. Is there anything else I can try, the piston and engine bore are smooth and in otherwise good condition.
