This sounds like the likely answer. There's no goo out the silencer, but I have some coming out between the cylinder and the pipe. The bike was running strong up until now. I don't have much experience with liquid cooled engines with power valves, I am used to old air cooled engines with no power valves. My Can-ams and Husqvarnas seem to love a ton of oil (I normally run 15:1 to 20:1). As long as they aren't fouling plugs, there hasn't seemed to be a downside. And the first couple tanks through the KX500 I was running C16 (117 octane) mixed 20:1. The engine ran awesome on this mix for 2-3 tank fulls, but it sounds as though its going to need a cleaning now because of it. Can I get at these parts with the cylinder on the bike, or do I need to pull off the top end?