Fuel problem finally resolved.

[hdavy]

After changing coil, plugs and fuel pump twice without success I finally replaced the jet. Bingo!
Think probably the little pipe from the float chamber and/or the little washer was at fault. Possibly suffering from modern petrol. Thanks to all for suggestions. (I never did get the carb bell off because heat, oil, and hammering a a torque driver would not shift the screws)