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Unknown Noise

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I recently bought a 1998 yz125 and i realized it has a noise coming from what sounds like the top end. It sounds like it could be piston slap. It is barely noticeable when idling, but once i start moving it is pretty noticeable. It sounds louder at lower speeds but that might just be because the engine is quieter. The guy said he just put in a new top end and i looked through the exhaust port and it doesn't look messed up (I didn't look at the entire piston, only the portion i could see through the port). What could this be? Should i be worried? Thanks

Edited by 99rm80

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Yes, you should be worried. If he just put a top end in it, and its making those noises, you could have a bottom rod bearing going.

This was the symptom I experienced before I decided to ignore them, and hope it went away. The problem never went away, and whilst riding down the trails, the rear tire locked up and the bike immediately died.

I remember that rattling, thinking to myself..."self, we may have a problem...lets get back to the truck..." Upon investigation, needle bearings in the crank went causing me to hang a ring in my exhaust port, locking the motor up.

Luckily I had a spare barrel, and found an OEM crank cheap.

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There is really no need to worry but I wouldn't run the bike until you take it apart and figure out whats going on. Don't trust that the guy did what he said he did or he could have also put the wrong size in, you'll never know unless you take it apart and see.

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I never do trust what they say but the fly wheel doesn't have play in it so could it still be the bottom end? or should i only be concerned with the top end? And just to let you know i didn't hear the noise when i bought it (2 days ago) because he had his truck running :smirk: and i haven't ridden it for more than a few minutes since i bought it so i know i didn't do anything to make it sound like this.

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"I just changed the top end..."

Unless you witnessed this, dont believe it.

We joke here that every used yz was NEVER RACED and HAS A NEW TOP END.

In fact, my gravestone will say....."***?? I just had a new top end"

Also, under many conditions, 10 hours on a 125 is worn....15 may mean worn out.

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Play in the flywheel wouldn't indicate a problem with the rod bearing, up and down slack in the rod once you have the top end off would.

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yeah i realized that once i said it :smirk: I guess getting into the top end is the only way to figure this out.

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"I just changed the top end..."

Unless you witnessed this, dont believe it.

We joke here that every used yz was NEVER RACED and HAS A NEW TOP END.

In fact, my gravestone will say....."***?? I just had a new top end"

Also, under many conditions, 10 hours on a 125 is worn....15 may mean worn out.

+ 1 million.... always pull the top end on a used bike, no question! If you dont have receipts or witnessed the piston installed in the bike dont beleive it.

My rule of thumb, always replace the piston, and inspect the cylinder, rod at a minimum when buying used. After convincing my buddy to replace the piston on his RM 125 that I know without a doubt had well over 100 hrs on it, I was surprised to see that it looked like the piston only had 5 mins on it when I pulled it out. Perfect condition... but that doesnt mean it wasnt worn past tolerences and be on the verge of catastrophic failure!

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well i just had a guy down the street who's a mechanic check it out and he said it's just the aftermarket pipe making it sound more pingy. I think I should be good to go. :smirk:

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well i just had a guy down the street who's a mechanic check it out and he said it's just the aftermarket pipe making it sound more pingy. I think I should be good to go. :smirk:

Pingy exhaust.....piston slap. Thats like saying Elvis sounds like Bon Jovi. Is he a bike mech or a truck mech....big difference. :smirk:

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The guy works on his dirt bikes all the time and he also works on trucks so he knows what he is talking about. I think i was just over reacting to the noise i was hearing because he said it sounds good.

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I agree with dgcars. Pingy exhaust and piston slap sound nothing alike. Not to mention pingy exhaust could mean detonation, if it sounds like piston slap it could be your needle bearing, piston worn past tolerances, bad rod/ bearing, or even a worn cylinder. Not worth it to not check it out because joe blow mechanic said your pipe sounds pingy. Idc how good of a mechanic you think he is, if he didnt pull the top end he has no idea whats going on inside, plain and simple. I do ALL labor on all four of my bikes and I cant begin to tell whats going on inside a top end just by a sound. It need to be physically inspected.

When it goes boom dont claim you wernt warned.

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I do understand what you guys are saying though. I should take the top end apart to be sure and yea i know he can't know for sure if there's a problem or not by listening to it.

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I am almost positive that this noise is only happening when i am in low rpms. The only time i hear it is when I'm bogging and it isn't there 100% of the time. When idling I can hear it very faintly and I wouldn't even hear it if I wasn't trying to. Just thought this extra information might help a little bit, it probably won't but it's worth a shot.

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Bearing noise will be least apparent at idle.

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record a video so we can try to hear it.

My camera is broken, i would if i could.

Bearing noise will be least apparent at idle.

I did not know that. I guess it's time to open her up.

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