The intake valve on #5 is either bent or has something caught in it. I'm with Steve on this - pull the head. It could be a chunk of a broken spark plug or something that got sucked in through the intake manifold. I vagely remember a Mud thread about some bolt in the carb coming loose and ending up in a cylinder. This might be similar.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln