Strange knocking sound when idling (yz 125)


17 replies to this topic
  • merrick

Posted April 13, 2009 - 09:03 PM

#1

I've got a strange sound when idling. Its coming from the cylinder, i think. I've changed the piston+ring+bearing, crankshaft+bearing, main bearings, and just renovated the cylinder. The sound disappears when i rev it just a little bit over idle. But comes back when the rpm lowers again. I'm running 98 fuel, with 3% oil in it. The engine runs really great, its just the sound thats concerning me.

What can this be, and what should i do to get it to stop?

Thanks!

  • AV8R

Posted April 14, 2009 - 07:09 AM

#2

Sounds like piston slap or noise that some describe a Wiseco piston making.
Since you replaced the piston it is not a worn piston so it would have to be the cylinder. Did you measure the bore before installing the piston?

  • merrick

Posted April 15, 2009 - 02:25 AM

#3

Sounds like piston slap or noise that some describe a Wiseco piston making.
Since you replaced the piston it is not a worn piston so it would have to be the cylinder. Did you measure the bore before installing the piston?


No, i didnt do that. And thats kinda stupid, i know. But if its piston slap, shouldnt it continue even at higher rpms?

  • JS264

Posted April 15, 2009 - 06:00 AM

#4

Does it go away if you pull in the clutch? Every bike I have had made a knocking sound at idle from the drive gear on the crank and the clutch gear meshing. Aluminum skid plates also make it quite noticeable.

  • AV8R

Posted April 15, 2009 - 06:18 AM

#5

No, i didnt do that. And thats kinda stupid, i know. But if its piston slap, shouldnt it continue even at higher rpms?


Piston slap is usually heard at low rpms and then dissapears with rpms, if it is not too bad.If it gets real sloppy then you will hear it at higher rpms.
Did you go from an OEM piston to a Wiseco piston?

  • gmoss357

Posted April 15, 2009 - 06:27 AM

#6

could be low end lean jetting. raise the needle by dropping the clip one position to see if that helps, or turn the AS in to see if it helps. If not, you have something mechanical going on.

  • harrperf

Posted April 15, 2009 - 08:19 AM

#7

sounds like piston slap....

You will probably be ok, but I would highly suggest tearing into the motor at 10 hours and inspecting.

The problem area is usually on the exhaust side right at the exhaust bridge on the piston..it slaps that and cracks, then eventually chunks. So look there for wear and small cracks.

  • AV8R

Posted April 15, 2009 - 08:21 AM

#8

Just re-read your post. You say your running 3% premix. So that would be 33:1. Try 26:1 on a gallon. My thought is if it is piston slap you will hear a difference due to more oil. Could be it's not even slap I'm just guessing without being able to hear it and given that you have replaced the items that may otherwise cause a knock.
Like J-rod said could be clutch noise.

  • Budlite

Posted April 15, 2009 - 04:55 PM

#9

If it were mine, I would pull the clutch side cover and make sure the final drive nut is tight. If it is loose, it will knock.

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  • sam2007

Posted May 06, 2009 - 10:22 AM

#10

did anyone find where this noise comes from ... i have the same noise ...i think it may be the crank main gear and clutch gears clearence ect .. this noise i have happens at tdc ... to lazy to pull the side cover .. really noticeable knock at idle and when kick starting ect. would be nice to hear from someone who has sucessfully diagnosed or fixed the noise /knock 2005 to 2009 yz125

better to find the noise before it becomes a problem ..my bike is currently running great and 2 hours on a new top end and bottom end the noise was there before rebuild also ... its just slightly louder now

  • mxrocks98

Posted June 15, 2009 - 02:52 PM

#11

did anyone find where this noise comes from ... i have the same noise ...i think it may be the crank main gear and clutch gears clearence ect .. this noise i have happens at tdc ... to lazy to pull the side cover .. really noticeable knock at idle and when kick starting ect. would be nice to hear from someone who has sucessfully diagnosed or fixed the noise /knock 2005 to 2009 yz125

better to find the noise before it becomes a problem ..my bike is currently running great and 2 hours on a new top end and bottom end the noise was there before rebuild also ... its just slightly louder now


I just did a top end on my new yz125 2005... i hear a little knocking(not sure exactly where it comes from) but just at idling, as soon as i touch the throttle it goes away. So my guess is that its just normal...but i would check the piston ansd cylinder after 10 to 15h too look at it and see if its still ok:thumbsup: and also make sure the reeds are ok... sometime's when there broke they make a knocking sound.

  • SuperHusky125

Posted July 05, 2009 - 02:16 PM

#12

i have the same problem with my husky 125 when its idling it makes a strange knocking noise and then dies on me, but thats only when its idling! when im riding its fine, i dont think its piston slap because you would hear the noise constantly, if any1 figures it out let me know aswell! thanks.

  • matthew trachsler

Posted July 05, 2009 - 02:25 PM

#13

did u get the cylinder deglazed before putting the new piston in maybe that would have something to do with it

  • Helmet_Hair

Posted July 05, 2009 - 05:23 PM

#14

To the original poster: what year is your 125? Is it air-cooled? Pretty common to hear a "rattle" at the cylinder that goes away when you rev it a little. My old IT did it almost from the time I put in the new piston with a fresh bore and hone. It's is quieter though than when the old piston was in. I saw scoring on the piston skirts and in the bore when I replaced the originals. They had 30-40 hours on them. My new piston and bore has only 6 hours (4 of those hours was an enduro) and it rattles like a paint can at idle. No noticeable drop in power or compression. Hope that helps.

  • YZ250Ryder

Posted July 27, 2009 - 10:24 AM

#15

A compromised head gasket seal on a liquid cooled engine can cause the knocking noise you are describing.

  • sdjclevland

Posted July 27, 2009 - 11:17 AM

#16

You know i heard the same thing after my 125 had been idling for a while just sitting there. It was not very obvious though and it was only when the bike was idling very very low. i assued it was normal though cause when its idiling normal its fine. it only did it the one time but usually i dont let it idle for that long

  • YZ250Ryder

Posted June 11, 2010 - 08:50 AM

#17

I have now installed a Fatty pipe on my '85 YZ250 and the noise is even louder! But searching threads on TT has brought me comfort that it is indeed normal. The sound, a steady loud knock, is very audible at idle, disappears with throttle and, very importantly, is rythmic, not sporadic.

Rest easy.:thumbsup:

  • brentn

Posted June 11, 2010 - 10:57 AM

#18

Piston slap IMO is more like a chattering noise if you ask me. You can tell if you have it if you run the bike in first at almost idle speeds, you'll hear a chatter indicating piston slap. At least this is what I think is happening with my bike.

The piston itself will have wear marks on the skirt in each side where it extends the longest. Usually a new piston pin bearing and a properly honed and sized cylinder will fix the problem.





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