Thank you Bernie. I have substituted the loose brass float in the new carb I bought and replaced it with the old plastic float that is attached to the lever. This seems to have cured the problem although I ave not taken it out on a long run so that everything gets really hot. I have also replaced the main jet with a smaller size and increased the air correction jet size both without detriment to the running of the engine. In fact the engine will now increase revs without hesitation.
Joseph Dobson