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ATK Rider 250

Piston slap causing engine knock

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I picked up an '06 YZ250 today and it had a significant knock in the engine. I asked the guy if he ran premium because my '02 would knock really loud when running anything less than 95 octane. He said he did but I'm not sure if I believe him.. Just out of my own curiosity I torn down the bike and top end right when I got home.. It has a hod rod crank and the piston had significant wear and side to side slap/ play in the jug. Even though it still started first kick when I was looking it over before purchase and felt the compression with my hand which "felt good" I still think the top end needs to be done. How ever I am not sure if this is where the knocking is coming from. The bike would not stay idling even after fiddling with the adjustment screw on the carburetor. It also has a full FMF fatty system, stock reeds. I have not checked the jets yet... I can take pictures of the piston and jug if that would help.

What is a good test/ indication to tell whether my crank rod bearing/ main bearings need to be replaced?

Any help would be GREATLY appreciated so please give me any feedback, I open to all suggestions.

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check for up and down play in your crank, that should be a good indication of how long it's been on the bottom end. there should definently not be any noticeable play in the jug, so it would be the best idea to replace your top end and then check your gap afterwards. A toasted piston may very well be the issue with the idling... As for the knocking, it could be a lot of things... check your squish to see if it is an actual pinging from the motor or something external...

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I picked up an '06 YZ250 today and it had a significant knock in the engine. I asked the guy if he ran premium because my '02 would knock really loud when running anything less than 95 octane. He said he did but I'm not sure if I believe him.. Just out of my own curiosity I torn down the bike and top end right when I got home.. It has a hod rod crank and the piston had significant wear and side to side slap/ play in the jug. Even though it still started first kick when I was looking it over before purchase and felt the compression with my hand which "felt good" I still think the top end needs to be done. How ever I am not sure if this is where the knocking is coming from. The bike would not stay idling even after fiddling with the adjustment screw on the carburetor. It also has a full FMF fatty system, stock reeds. I have not checked the jets yet... I can take pictures of the piston and jug if that would help.

What is a good test/ indication to tell whether my crank rod bearing/ main bearings need to be replaced?

Any help would be GREATLY appreciated so please give me any feedback, I open to all suggestions.

Check the crank for up and down play, zero is what you need. check for rod play, also remove ignition cover and try to rattle the flywheel up and down, again looking for zero play.

its a used 8 year old motocross bike. Replace the topend asap with a oversize piston

stock bore is 66.40, stock piston is 66.35, by now the bore is well beyond the oem 66.40 to keep the optimal .05 piston to cylinder wall clearance you need a slightly bigger piston

pro x, oem , wossner makes stock size pistons as well as slight oversizes

66.35 A (fits perfect stock 66.40 bore)

66.36 B

66.37 C

66.38 D

rebuild it using the biggest oversized piston (oem D or pro x D)

or if you go forged (best option but a few more $)

wossner http://www.wossnerpistons.com/products/product-details/id/WOS-8216DC/name/wossner-piston-yamaha-yz250-66-42-mm-bore

Also, i would do this, use a new piston for this test.

http://www.rb-designs.com/squish-band-test.htm

if the number you get is larger than 1.5mm send it to rd designs toget the squish reduced and rechamberd for pump fuel.

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And yes there is a good chance the rattle/knock you hear is piston slap from excess rocking in the cylinder bore.

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