I can't diagnose a problem I am having in the driveline of my 2000 WR400. Last ride a vibration started. It results in a large clunk and the chain does a large slap onto the swingarm, when the bike is on a centre stand, each time the throttle is revved up and each time the throttle returns to an idle. It seems like a large amount of "slack" in the driveline somewhere. I have eliminated the primary drive gear nut being loose as well as the counterbalancer driven gear and key. This clunk and slap scenario repeats itself in all 5 gears The transmission shifts very smoothly and I have not had it jump out of a gear yet. I noticed quite a bit of free play when I rotate the mainshaft of the transmission before the countershaft starts to turn but I can't find out from the manual or from thumpertalk how much play is normal. Can any one help me out?