I have a Beta 80 with the same issue, killing a spark plug after a few hours riding with lots of black carbon deposits. I just spoke to Beta UK, they suggested using a BP5ES spark plug. Apparently the higher the number, the better the plug for hotter climates. If you're riding in the UK they suggested a lower number like 5 (rather than the 8 I was using) would be better for our weather temperatures. I hope that helps.