It could be too lean but if it was flown, landed then just put away it could be a couple of other things. As an engine gets older they start leaving a bit of gray mater flowing out the pipe, that's wear in the engine and it can short out a plug, little bits of left over metal. If you are using after run oil it too can get into the plug and short it out. You can try running your engine dry after landing and before you pack it up and go home without adding any after run oil. The .40s can be run from zero nitro up to 15% or even 20% if that's what you have. I run 15% in everything and don't give it any more thought. The plug of choice in most 40s is the OS #8 or the A-3, I have used them both but the #8 is what I run in every two stroke from .40 up.