Yz450f Help!!!!!!!!!!!

I have a 2003 yz450f, the bike has been great. I change the oil every other ride. But yesterday when i was at the track the bike ran good all day and at the end of the day on my last lap you could hear something raddling or klanking in the motor. It still runs strong and still has great compression, and the oil still looks clean. Please help I can break down 2 strokes all day but I've never broken down a 4 stroke.

Welcome to the site. Have you ever checked/replaced your valves? Yammi's are usually known for their valve reliability but if you've never torn it apart and its an 03 you should get those checked for sure for starters.

Sounds like the engine balancer gear key is worn and needs to be replaced. I had a 2000 YZ426 that developed a similar rattling noise. I replaced the key and that eliminated the problem. Search the Yamaha sub-forum and you should find a wealth of information. Good luck.
There is no such key on any YZF made after 2000. They went to splines in 2001.

Are you sure the rattling was not present before?

Are you sure the rattling was not present before?

yeah its never rattled before, i ran it up and down the alley last right and it still runs the same (strong)

If the rattling appears to be coming from the right side of the engine, open it up and check the nut on the crankshaft and on the mainshaft, holding the clutch.

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