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timing slipped? 2007 crf250


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I have a 2007 crf 250 it had a recent top end . I has about 12 hrs on it and it started to make a noise of the valves hitting the piston during a race 3 laps in . Shut it right down it wasn't like they were slamming it just barely hitting . Pulled it down and checked the timing it was off 3 teeth . What would cause this sudden jump or change ? I checked the auto tensioner and it is good .

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tensioner failure is most common, best to replace with a manual tensioner to prohibit any problems, also the cam gear may have spun (only pressed on to the cam), way to check that if the cam gear wasnt notched to the cam itself to tell if it did spin is to put it on a cam degree wheel and see where its at.. everytime i put a new cam in a bike i notch it so i can see if it has spun at all, other possibilities as noted before is the chain stretching out, stretching out that much is kinda excessive to me and rarely happens.. where in Nv are you?

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las vegas . Don't know how to degree the cam . I did mark it when it was apart and it looks like it did spin about 1 tooth but not 3 . Put a tack weld on it and set the timing back to my mark started up and seems good . I just want to know if anything else could have had an effect because I don't want a fail at a track again .

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