I assume you have the TN carb on your Webra 40. You need to make sure the barrel gap is small when carb is closed. Using a small screwdriver screw in the low speed needle all the way and backout 2 full turns. This sets the cone (mid range adjustment). Start the engine, let warm up and bring to about 3000 rpm. If the revs fall off the mixture is to rich so adjust in small increments ti 3000 rpm is held. My Webra 50 GT didn't require this but my Webra 40 GT did. It works. These are good engines. I also use that prop, that plug and that fuel so it is not the problem even though the engine is most likely ABN. Good Luck!!! Do you have the white copy of the Webra instructions?????