Exactly which engine do you have?
40 LA
40 FX
Something else?
You should be able to download the manual from http://www.osengines.com/manuals/index.html

I think the 40LA originally came with the A3 plug. OS recently renamed this plug to #6
I think the 40FX originally came with the #8 plug.

The #8 is a slightly "cooler" plug than the #6. It is possible that switching to a cooler plug will help. As others have written, I think your mixture is too lean for the fuel and plug combination. I use 15% nitro and the #8 plug in my two stroke engine and the plug lasts a long time.