why are all of the pistons dirty accept one?
when i pulled the intake mani the second pipe on the mani was pretty dirty down where it connects to the head.

the intake valves are second to the right in this next pic. not sure what the exhaust valves look like, theres one last bolt i cant get off the mani
on the valve side of the head the valves for piston 2 were basically clean

what could've caused this???