Sorry ptg,
Did you check the screen in the carby as it has a very tight mesh and any dirt will block it making it hard to start.
Also check the screws holding the mounting block to the cylinder because if they are loose you will also have prob's starting.
As to the plugs I prefer to use NGK as I have found them to be reliable.
Another thing would be the carby settings on the high and low mixture screws.
Let us know how you go after checking the above as well as the mixture settings.