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Please help I accidentally took this piece out of the side of my engine on a 2004 yz250f and idk what it is I tried putting it back in but now I can’t kick my bike.

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I think it's the camshaft chain tensioner.. Be sure to put it back in before running the bike! 

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yea, timing chain tensioner.  If you tried kicking the bike over with that out I would pull valve cover off and be sure it is still timed correctly.

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It’s definitely the cam chain tensioner. As others mentioned you need to check timing before trying forcefully to turn the engine over. 
 

but to reinstall the tensioner you should take the bolt off the rear middle. Then use a screwdriver to reverse the tension pin. Then reinstall. After it’s installed release the tensioner because they work automatically and lock from allowing the cam chain slides from pushing back

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 As stated above, that is your timing chain tensioner. It must be reinstalled in a certain way to work correctly. If you simply stuck it back in the hole and somehow (I can't imagine how) got the mounting screws back tight then your timing chain would be WAY over tightened and that would be why it won't kick. 

 The tensioner auto tightens. So when you pulled it out it sprung outward into fully tight. It must be reset. You need to remove that single screw on the end of the 'cylinder' (below your thumb). This is a cover. Inside there is another screw that you need to get a very small screwdriver and fully  loosen. It is under spring tension so once it is all the way backed out then you let it ease back in and it is then "auto set". Your other problem, I have to imagine, is the tightening torque on the two mounting bolts. Ordinarily you would tighten those two bolts while you have the inside adjuster screw turned all the way back (fully loose). So check that. if those bolts are not correctly tightened then it will leak oil. 

 One other concern is that if you forced the tensioner back on the you could have potentially damaged the plastic chain guide that it pushes against. The good news is that the guide is pretty tough - so it may be okay. the bad news is that the only way to know for sure is to remove the head and visually inspect. 

 

 Looks like Deetle15 beat me to the punch.

Edited by 2FlatsandBent
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^ this contains everything I didn’t type in my reply. Being on mobile and kinda busy I opted for a fast simplistic reply.

 

but good luck OP hope you figure it out with minimal headaches.

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I'm guessing the bike is new to you and didn't come with a service manual.  You are probably new to servicing a four stroke as well.  Invest in a good service manual before removing anything else from the bike other than the gas cap.

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I think you took the cam chain tensioner out completely.  Then replace it back without undoing the middle screw.   So the tensioner is all the way out.  You have max tension on your chain.  You have to loosen the middle screw.   Then take a flat head and undo the tension from the middle screw.    While holding the tension out with the flat head.   You need to replace the tensioner on the bike and assemble.   After the tensioner is in and both bolts are tight, you can now unscrew the middle bolt tension.   Then tighten the middle screw on the tensioner.   

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3 hours ago, donwann said:

I'm guessing the bike is new to you and didn't come with a service manual.  You are probably new to servicing a four stroke as well.  Invest in a good service manual before removing anything else from the bike other than the gas cap.

If he had a manual and read it.... he would have not unbolted the tensioner in the first place🤯.  He is here looking for easy answers and we all know  this huge area of confusion and far from easy.

 

Edited by TX-SANDMAN

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On 10/10/2020 at 2:43 AM, John3463 said:

Please help I accidentally took this piece out of the side of my engine on a 2004 yz250f and idk what it is I tried putting it back in but now I can’t kick my bike.

image.jpg

To be honest, if you need to ask then you’re in too deep.  This tensioner is very important, if you get it wrong it will detonate your engine.

Most mistakes you can use as learning curves but not this one.

The fact you have discovered the engine won’t turn over means you are too late and already in trouble. Get somebody local to help you who knows about bike engines.

 

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I was once told by an older wise man. We pay for our education. Whether you go to a formal school, repairs using process of elimination, (backyard, backwoods, back 40 acres technology), your OJT big brother, dad and friends teaching you etc. We all pay for our education. My parents called 90% of my educational mistakes "live and learn".

John3463, you have now stepped into the TT World of live and learn. You are beginning to pay for your education, whether is be corrective criticism, words of TT wisdom, buying a manual before wrenching on your bike & following it or just junking it to sale the parts. I suggest being patient and take the good advice posted here and do your best to shrug off what you consider BS. As in all TT Topics where someone like yourself buys/owns a dirt bike just took stuff apart without looking at an OEM Shop Manual or Equivalent. Like a few TT'r's before me commented "buy an manual" they really do help and like Hughie posted find someone who knows how to help you recover from your mistake.     

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