[GavinCarraway]

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.

 

[GavinCarraway]

The new top end I put on consisted of a new cylinder, new piston and rings, and I went ahead and replaced all the valves and valve springs.

 

[GavinCarraway]

I did two cycles where I let it idle for 15 minutes and let it cool each time in between. After that I rode it really easy in low rpms for around 30 minutes. Once i did that i began to slowly start to open it up for about an hour period. After that i rode it normally. I used Amsoil metric 10w40.

 

[GavinCarraway]

No I did not let it idle longer than a minute. Like I said I'm new so I never took the measurement of the piston to cylinder clearance.