Great post by Yetti831 and also good info from fly x. When Gareth explained the original situation he mention that is a 10 year old engine which I think that was stored. In my opinion I would tear apart all the engine and specially the carb. On the engine I would look for corrosion (even that this is not the problem, it could be in a very near future), once the engine is cleaned up (Changing bearing could be a good choice here) I would replace all O rings and make sure that the carb is completely clean (10 years of stored castor oil would produce a lot of mess inside). Then assemble the engine again and run it in a test stand (If available) and start tuning the engine just as Yetti831 described, at the end you will have a great performer.
I have stored a number of engines for a while and I had to replace o rings and flush the carbs until they are completely clean, then they perform just like the last time that I use them. Now a days when I know that an engine will be stored then I take the glow plug off the engine, plug the carb, fill the engine with transmission oil and put a dummy plug, that simple procedure lets me sleep knowing that all my engines are well cared.