I’m new here and I don’t have a lot of experience working on engines but I’m curious to what could have caused my 400ex to do this? I read that It running lean could cause this but I’m not sure due to me not having a lot of experience. It was a brand new top end that I put on. Made sure the rings where all good before I put it in as well according to wisecos specs. I did the break in on it and then rode it and found out it was definitely lean due to it having a aftermarket pipe and all. I put a bigger jet in it after the ride. Rode it the next weekend and then this happened so not many hours on the engine (maybe at most 3). Would love some opinions on what would have caused this so it won’t happen next time. Thanks.