I think the grinding sound was actually the starter motor that did not disengage properly. That seem to have cured when I put a better battery in, or was it just a co-incidense !?
I put a new fuel filter in and replaced a slightly split fuel hose. I also checked there is a good flow coming from the fuel pump.
The latest trial did not show signs of misfiring anymore but it did not rev at all. there is this hollow sound coming from the air file - like it is sucking air but not firing (bit hard to describe). there was also what I think traces of unburned fuel down on the grass behind exhaust pipe. I unplugged the cold start injector and that did not change anything.
I am sure it run much better on Saturday when I first started it. On Saturday it was really misfiring for first few minutes but then came alive suddenly. Maybe I need to drain the fuel tank next in case it is just old petrol, or water in the petrol - we had some heavy rainfall couple of weeks ago.
To be continued, I guess...