To able you help you more, we here need to know what carb is fitted to the engine to see if any settings can be done to it, post up the make and model numbers, then give us a little time here and am sure someone will get back to you.
running a engine too lean can cause problems! engines that run too lean run hot, this isn't usually the kind of hot that you can see on your temperature gauge its mostly hot inside the cylinders and the exhaust system, when engines run too lean damage to the pistons and valves can be the result, pistons which get over heated and distort and melt and burn and basically cause lots of problems with compression and burning oil, lean conditions can also cause the exhaust valves to over heat and distort and not seal properly. A valve needs to get a good seal not only to hold compression but also to cool it self, heat is taken away from the valve while it is closed, the heat goes into the valve seat and then into the cooling system.
Serious damage from extended periods of lean running are common and sometimes are hard to spot.
