MMMM I wonder if that screw is far too loose? and it should be tightend.? Carby screws should not be that loose! My theory being that if cracking or advancing the throttle to get it to start is actuating the main fuel circuit as there may be too much air getting past this screw if it is part of the idle air or fuel bleed circuit, there fore delaying activation of the fuel into the engine beyond resonable limits.
When the engine fires up does it appear to be rich? i.e. is their black smoke from the exhaust or is it "clean"?
This should give you a few clues, but from reading it seems like a lack of fuel
If you find a sweet engine starting point and the screw is still "loose" raid the wife's fingernail paints and paint plenty around the screw to hold it right, this may even stop any other air leaks if this is the trouble.
