Nick.
Look for this post:
Author: Ron Brown (---.ismi.net)
Date: 07-20-01 11:58
It is quite long so I do not want to repeat it.
I would recommend that you do not drill the carb because of the problems with the throttle slide, but the rest of it may give you some ideas. Mostly, I suspect that you have not really cleaned the carb well enough. Remove the main jet from the holder and examine both with good light and a good magnifier to make sure you have no crud left in them.
Ron