04 YZ 250 Knock

C

Chris

Noob
I have a 2004 YZ 250 with a brand new rebuild (top and bottom end). There are virtually no hours on it and I noticed at idle, and ONLY at idle there is a knocking. Some of my buddies I have asked thought it could be the crank bearings but as I said everything's brand new. Wiseco crankshaft kit is what is in it now. When revved or in gear there is no knock. Runs perfectly fine other than hearing a knock at idle. If anyone has any suggestions I would appreciate it.

Thanks.
 
hershey said:
Probably just the clutch. Does it change when you pull the clutch lever in at idle?
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My 2006 YZ 125 knocks until I pull in the clutch. Time for new clutch everything again in the the most fun bike ever.

Oh Mr Bill, look what I found in the almost brand new Mikuni KTM take off that finally fixed all my weird engine issues.
IMG_20230514_141227790~2.jpg

The original YZ Mikuni carb body was just plain worn out. But a swap of all the upgrades from the old carb to the new made it right again. Now it's clutch time.
 
Is this normal? I’ve personally never seen this much play with a piston before but this is also my first Yamaha.
 

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Chris said:
Is this normal? I’ve personally never seen this much play with a piston before but this is also my first Yamaha.
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I hear bad sounds but don't see anything in that "video".
Here's what a bad main bearing sounds and looks like. This was my 1996 360 Husky before repair. It had a knock as the flywheel moved in many directions. Unfortunately the flywheel magnets are damaged also. Does anyone happen to have a nice one on hand? Lol, not for this rare bike, YZ parts are much easier to find.



 
k01 said:
I hear bad sounds but don't see anything in that "video".
Here's what a bad main bearing sounds and looks like. This was my 1996 360 Husky before repair. It had a knock as the flywheel moved in many directions. Unfortunately the flywheel magnets are damaged also. Does anyone happen to have a nice one on hand? Lol, not for this rare bike, YZ parts are much easier to find.



View attachment 79235
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Here’s the video not blurry. Not sure what happened on the upload.
 

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Had the same problem with my yz250.
Turns out I had the wrong size piston, which was causing piston slap/knock.
like Chris was saying, make sure you have the correct size piston for you’re cylinder.
 
k01 said:
I hear bad sounds but don't see anything in that "video".
Here's what a bad main bearing sounds and looks like. This was my 1996 360 Husky before repair. It had a knock as the flywheel moved in many directions. Unfortunately the flywheel magnets are damaged also. Does anyone happen to have a nice one on hand? Lol, not for this rare bike, YZ parts are much easier to find.



View attachment 79235
Click to expand...
Yes, thing is knocking with the piston rocking. Not much resistance, did it rev quick and have little compression?
 
Cylinder's need replated from time to time to bring them back to OEM spec.
I've had to send several out due to wear.
 
If you have an aftermarket clutch basket make sure the gear on the back is secured properly. Ive seen these bolts back out and that can create a knocking and catastrophic failure eventually.
 
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