I might be totally off base, but I had very similar problems with my 100. I thought for sure I was just too lean, but the plugs were black. Turns out it was overheating due to the design of my cowl - all the air was running under the engine, and not up and over the cylinders until I put in ducting. These things will store heat for a long time inside a cowl, have you tried running it without a cowl?