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So I recently bought a 2008 400ex. I noticed a ticking noise coming from the motor. Then I listened and heard a weird noise when giving it gas and then letting off. It does have a full fmf exhaust. I don't know if it makes a difference. But I was told no motor work has ever been done to it. I'm at a loss here. I have a video so you can hear the tick and the noise it makes. https://youtu.be/5ixm3Ud0Uko
 

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From the video (at least to me) it sounds like cam chain slap due to a worn cam chain or the cam chain tensioner is stuck and not taking up the slack

Put the engine on TDC of the compression stroke, remove the bolt from the center of the cam chain tensioner, loosen the bolts holding the tensioner to the cylinder by about 1/4 inch each. If the tensioner does not move away from the cylinder under spring pressure as the bolts are loosened, tap the tensioner with a screwdriver handle. If the tensioner does not push away from the cylinder under spring pressure, either the chain is worn out or the tensioner is stuck.

At this point, remove the tensioner (it should be fully extended)(measure how far the plunger is extended from the base of the tensioner body), using a small flat blade screwdriver, insert it in the hole on the back of the tensioner and turn clockwise to retract the tensioner plunger - when fully retracted, release - it should extend fully, smoothly without catching - to test the full extension, insert the screwdriver and gently turn counterclockwise - if the plunger fails to extend further when turned with the screwdriver, measure the plunger extension the same way you measured it the first time - if it is the same distance, presume the tensioner is working and has reached the limit of it's travel and the cam chain is worn out - if the plunger is out further than when first measured, retract the plunger with the screwdriver, install into the cylinder, release the plunger, install the screw plugging the hole, restart the engine and see if the noise is gone. If the noise is still there, plan on replacing the cam chain.

You can go ahead and ride (even though noisy), but plan on replacing the cam chain SOON! or better yet, park it till the cam chain is replaced.
 
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