JCB 4855F keeps stopping?

It starts but then runs and splutters for a few minutes and stops then it will not start. It seems to be running rich, the plug is a bit cokey. It is about 3 years old but has hardly been used. Carb is full of fuel, float works, air cleaner is seems clear, no leaks, complicated arrangement of levers and springs a couple of lugs have holes that look like a spring should be attached but isn't there? anyone got a photo to compare. It is not clear how to adjust mixture. Anyone have an engine manual? It says OHV on the front no brand (apart from JCB)- I think it is a Honda Copy. Vpoorly engraved number that I thinks says JF1P84FJV 0701291015. All the usual things tried - result - I can get it started again but then the same thing happens. Any ideas?