Where i work, a 1998 WR400 came in, initial thoughts were, possible broken valve. It had no compression, and you could turn it over a couple of times, then it would lock up, jiggle the bike about a bit, and it would turn over for a while then lock up again. On further inspection, after removing the head, we found two lumps of metal, one about 4mm x 4mm by 0.5 -1 mm thick, the other was about 25-30mm long by 4-5mm wide, and again 0.5-1 mm thick. We were bemused, what the hell were they??? On inspecting the carb, all became clear, the baffle plate that can be removed, had the bottom corner missing, this had obviously been disturbed/cought/broken etc, it had then travelled into above the piston. Thankfully, i minor miracle i think (not litterally) neither the valves, nor piston has been damaged atall, lucky i say. So, if i was you, i would check your carb slide baffle plate, bacuase they look quite thin and weak compared to the modern FCR carburettors found on the newer bikes. Just thought id share this info with you, just as a precautionary measure, .