Purchased a non-running 1998 YZ400F, engine would rotate until very nearly TDC and stop. Disassembled the top end and found the right intake valve had broken and became lodged in the top of the piston. The cylinder somehow didn't sustain any damage. On disassembly I noticed it has a hot cams intake cam in it. The top of the piston and cylinder head were dark like it was running a tad rich maybe, no sign of overheating. Are these bikes prone to valve failures?