YZ426f engine noise -light rattle or nock?

i just puchased a used 00 yz426f. It is in great condition. new parts and well maintained. when i looked at it i could not find any leaks or damage and did not notice any problems. It starts right up, runs good and has plenty of power. The previous owner said he had recently had the cam chain replaced. For the few minutes i looked at it everything seemed ok. KBB was listed at $2260. I paid $1700.00 it has some after market goodies and extras with it. I thought this was a good deal so i bought it. When I got it home it started up easily once you know the "routine" and I ran it around for about 30 minutes. then when it was nice and warm i was listening to the motor and noticed a rattle or light knock coming from inside the cylinder. I checked elsewhere for any loose or vibrating parts and found none. The noise is not coming from the top, i dont think its the valves. It sounds like the wrist pin on the piston/ rod is vibrating or loose. Or a bearing? i dont know. Has anyone had this problem before and what do i do?:banghead::busted:

First, there has never been a YZ4** that didn't rattle. But if you think this might be abnormal, the 2000 models do have one thing they like to do that causes a rattling noise in the right side of the lower engine.

Check the nut that holds the primary gear on the crank. If this gets just a little loose, the balancer shaft drive gear starts rattling on its drive key. It's no big deal if you catch it early, but if you let it go, it can be expensive. If it's loose, replace the key.

My friends 01 YZ 426 had a pretty good rattle or knock that started all at once & it ended up being his wrist pin bearing. That was after 5yrs on the stock top end & his questionable maintenance record. Try that on any other brand of 4-stroke!

ditto on the counter balencer key---rebuild a 01 model and a week later it had a bad knock---replaced gear and key and cam chain just incase---fixed that noise

Thanks for the info about the keyway guys, I will definately look at it. I am going to get a manual for the 426 before i get into it.

I also have a 2000 YZ426 and know excatly what your talking about. My advice is to ignore the issue after you check the key on the primary gear. I know it might sound hard to do, but my bike is still running strong after 3 years of punshiment in the sand and tight wood trails. Eventually you will get used to the rattle. Good Luck...It's a great bike...

for what its worth i have a brand new WR450f and it rattles as if its full of rocks (lol) this condition is normal due to the use of light weight materials.....light,high performance engine doesnt equal quiet....these bikes are built to be ridden hard.....and no attention has been paid to noise dampening....great,light,reliable bike...just noisey

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