06 yz450f dropped valve

Ok so here's my story i was out riding my bike one day and then all of a sudden it just seizes up locked the back tire up, anyway so i bring it home and start tearing into it i take off the head and well the every thing was screwed up, one of the intake valves was stuck in the head and the piston had decent sized chip in it from the valve. I'm guessing it jumped time and the piston came up and hit the valve and broke it. anyway i know it wasn't the timing chain because it only had about 20 mins of riding time. I don't know why its jumping time i don't want to fix it then just have it happen again, what do you guys think?

Check to see if one of the cams has seized in the head (look for galling in the cam saddles/shafts)

Well I checked the cams and they look and rotated as they should. here's a pic of the the head and cams.ImageUploadedByThumper Talk1374016043.230906.jpgImageUploadedByThumper Talk1374016057.399434.jpg

If they spin fine it could be worn cam and or crank sprockets or a Defective tensioner. Test the tensioner and see if u can push it back. How was it running before this happened?

Occasionally, a cam sprocket will slip. This is often obvious visually. Line up the timing marks on both cams as if the chain were in place, and then take a picture of how the marks are lined up, and another picture from the other side. Let's have a look.

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