So my buddy just picked up an 06 yz450f. We were riding today and all of a sudden it started blowing blue smoke out the exhaust. A pretty decent amount too. I know that means it's burning oil, but what I don't get is why it is burning oil. Supposedly it has 6 hours on a full rebuild- top and bottom. We took it back and drained the oil and took the filter out and there was some metal shavings on the filter. The oil was pretty dirty and dark gray as well, so it must not have been changed since the rebuild. What's weird is it still runs just fine. Starts easy and idles perfectly. There is a little bit of a knocking noise, but it sounds like the decomp valve. Not 100% sure though. Could it be the previous owner not breaking it in correctly or what? I'm confuzzled.