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Help with engine noise.

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04 yz250f.Motor top end was fresh as of winter.About 40hrs MX(no pro here) track time on the bike with absolute minimal limiter hits.I was riding up a long hilltop step up jump hard.The motor was running real strong all day.After I landed up top,I 180 turn back down the hill.I heard a bad motor noise and hit the kill switch.The noise I thought at the time was a broken cam chain adjuster.It did not matter if the clutch was in or out or in gear or not.I tore the bike down and found valves in spec,no scoring on the cylinder or piston,no timing skip,new looking timing chain,strong adjuster,no metal more than usual in the filter,clutch in VGC,and no broken rings.Drained oil looked good.This thing gets oil and filters changed every riding day without fail.Any ideas where to go from here?Anyone have the same thing happen?....The best way to describe the noise is similar to if the kick starter was held down while running or as I originally thought,if the timing chain was loose just slapping around.I thought the sound was from the left side of the bike,but I could be wrong.

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That's exactly what ours sounded like when this happened.

The kid over shot a 70 footer and blam. He managed to shut it down quick enough but parts of the spring made it all the way over to the flywheel side. We ended up disassembling the entire engine in search of all the broken pieces. We got lucky and nothing made it into any bearings,piston or gears, just some scuffs on the cases.

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Those springs were good...I pulled the basket out and felt the gears.One felt funny.I looked behind it and a metal part was between the case and gear.No damage luckily....The doophus who sold me the bike lost a wrist pin clip and topended it in the Winter.I guess it was just too hard to do it right and find the lost part.Well,I found it Sunday.I'm going to topend it as long as it's apart.Thanks for the help everyone.Looks like I get off cheap this time.

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Well,you tell me.He just rebuilt it this Winter.He was riding and heard a noise.He stopped and tore it down and he had lost the wrist pin clip and the piston pin had took out the cylinder side.He put a piston,rings,and cylinder on and sold it to me saying it was all good.I think he never found the clip(which is exactly the same size as the wrist pin clip)and probably just slapped it together.Everthing else on the clutch side case is there.The only other thing would be a clip on the other side breaking and moving across the lower end.I guess it's worth a look.It's about totally apart anyway,but I would bet that that was the old clip.Why so wierd that it could get down there?I had a clip in a 125 fall of the carb needle and go through the top and bottom end..........greyracer,the piston pins were still in the piston that just came out.I think he was lazy and did not want to split cases and dumped it.

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Finished it at 10pm last night and rode it today.Sounds/runs perfect again.I found no more debris in the cases and after looking at it you can see there is a easy passage for the clip to fall into either side of the bottom end...I would like to add that the four stroke rebuild was not as bad as you think.Most of the problems are getting the motor in and out of the bike.

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