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Ive been fighting this issue since last summer. 2012 yz 450F locks up when trying to start it. When I put it at the TDC, ( rarely ) it will lock up. Will not kick over. Last summer I found a stretched timing chain. Replaced the timing chain, chain tensioner, pulled the top end off looking for any indication of something catching and found nothing. It does this know matter if the engine is warm or cold.. The bike runs great and does not skip a beat so I am at a loss on what is the issue? Any help would be awesome. Fed up with this thing.

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I've read that on the older YZ's like my '05, the pin for the auto-decomp will get hung up and jam the valve train.  Not sure if the FX has the same style decomp or not, but when it happens, you're supposed to put it in gear and bump the engine backwards.  If it is doing it all the time, then perhaps the decomp pin itself is bent and/or damaged.  Take a peak at your valves and make sure none are bent and that there isn't anything binding.   Being that it runs fine once started, you likely don't have a bent valve, but doesn't hurt to look while it's all apart.

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Okay Im gonna take a look right now. Last year it did it all the time but like I said, I had a stretched chain. And has been good since I replaced it last year and it just did this again a few days ago. If I can find my dang 14 mm for the flywheel cover ??

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The pin doesn't jam the valve train, or anything else mechanical.  To the contrary, it's the pin that gets jammed.  If the engine is stalled in gear, it may come to a stop so quickly that the flyweight spring never has a chance to extend the pin.  The most likely place for the engine to come to a halt this way it right against the beginning of the full, non-reduced compression stroke, which makes them nearly impossible to turn over past that point.  The easy cure is to put the bike in gear and pull the engine backward just a little, which frees up the auto decompression works and lets it do its job. 

If you had a bad timing chain, and the timing ended up one tooth advanced, it would be very difficult to kick over all the time. 

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21 hours ago, grayracer513 said:

The pin doesn't jam the valve train, or anything else mechanical.  To the contrary, it's the pin that gets jammed.  If the engine is stalled in gear, it may come to a stop so quickly that the flyweight spring never has a chance to extend the pin.  The most likely place for the engine to come to a halt this way it right against the beginning of the full, non-reduced compression stroke, which makes them nearly impossible to turn over past that point.  The easy cure is to put the bike in gear and pull the engine backward just a little, which frees up the auto decompression works and lets it do its job. 

Quote

If you has a bad timing chain, and the timing ended up one tooth advanced, it would be very difficult to kick over all the time.

Which is why its weird. Last summer when it was truely jumping timing, their was a time it jumped 6 teeth and still started no problem, wasnt any harder to kick over. Which is why I was BEYOND confused why the motor never went down. But besides the point. Me and some buddies are going to silver lake sand dunes at the end of may. So im going over to his house to really take a close look at it. If I do not find anything wrong with it, I guess ill just let it be.

 

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You would have to show me an engine that was out of time by 6 teeth and still ran. 

Regardless, if you have the cam cover off, you can see what I'm talking about.  Put the engine at or close to TDC, the cam timing position.  Doesn't have to be exact.  Fold up a matchbook cover or a piece of business card, hold the flyweight on the exhaust cam outward so the pin is retracted in the run position, and tuck the card under the flyweight securely enough that it will stay there while you turn the engine over fairly slowly.  Rotate the engine forward by hand until it hits compression.  Notice where the pin is; it will be down against the lifter, and when you pull the card out from under the flyweight, it won't extend.  Just for fun you can try to kick it through form there and you'll see what I mean. 

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So I finally got around to looking at my bike last night. Valves were in sec and timing was on. Only thing I could find wrong after taking the cams out and taking a really close look was the decompressor sticks. That be my issue? I found on my last ride that when it would lock up when trying to start it, if I put all my weight on it. After 10 seconds or so it would free up. The pics are of my intake cams. What are the marks on it?

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so yesterday was interesting. Drain plug fell out while riding ? nice 2 hour push back... But I caught it before anything happened and shit it off. I haven't dug much into it, but what do you guys think? Should I dig into it more? Cams don't look like they have damage. Little discoloration but nothing you can feel to the touch.

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