the cold start injector will be squirting while you are cranking and then for a few seconds after. that will be why it runs for 5 to 10 secs.

I had a similar problem with mine a while back. I could get mine to run if I pressed very softly on the fuel metering head flap.

I ended up fitting a new black ecu and everything was good.

You've already done this so not really sure.

check the flap pulses while you are cranking.
also try adjusting the mixture screw clockwise to reduce how far the flap can be pushed down before you feel pressure.

also had problems with spark plugs in the past. mine don't seem to last long at all.

this may all be wrong but maybe worth a try.