Da Rock: Thanks for the step by step instructions. Its never been explained like that to me before. I can tell you now I did not see any smoke out of the exhaust so thats probably it. Thanx again.
ORIGINAL: Gray Beard
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.
I am using 15% and its a new engine. I got it in the box, but the instructions were missing. The plug looks like it has a 6 on it. but the stamp isnt very clear so it might be an 8, I just cant tell.
Thanx again to all that have expressed their thoughts. Ive learned quite a bit today.