2000 yz125

[eagles22793]

I have a 2000 yz125 with a knock in the top end only at idle. When riding the bike I do not hear it. I did a compression check and it is higher than stock and a piston kit was just put in it with very low hours and looks to be a new one in it. I did not do the piston kit the guy before me did. I am going to the track tomorrow and I am going to use ams two cycle and not the cheap stuff from auto zone and I hope this will fix it. Has anyone had this problem and what fixed it or what can I try to fix the knocking sound?

 

[MikeT]

Different oil will not fix a knocking sound. If you have a knocking sound, I'd remove the cylinder and piston and have it measured by a reputable shop to check the tolerances. You might also have a bad wrist pin bearing or bad crank. If the piston and wristpin check out after being measured, then you need to look at the crank.

 

[eagles22793]

I went to the track twice and have not heard any difference in the knocking sound after using ams two cycle and 40:1 mix. I was at the track for a total of about 12 hours and I think if it was the crank it would of came apart or lost some power. It seems to run great just at idle it has a knocking sound. I dont want to rip the hole engine apart for nothing. Is there any thing I can check to makesure it is something in the top end before ripning it apart

 

[rmc_olderthandirt]

What are you using for gas?

If the compression is significantly higher you may need to run a very high octane "race" gas.

If I was you I wouldn't wait until the engine comes apart before you do something. Tearing it down now will only cost a set of gaskets it it really is for nothing. Waiting until the engine grenades can turn the engine into scrap.

Rod

 

[Rich Rohrich]

What are you using for gas?

If the compression is significantly higher you may need to run a very high octane "race" gas.

Rod - It won't spark knock at idle even with poor fuel. If it's knocking at idle it's from something mechanical.

That's not to say he won't frag the thing from combustion knock due to the wrong fuel once he runs it hard with the extra compression. It seems like people who wait to "hear" spark knock usually find out about it too late. ;)

 

[2strokerfun]

You will eventually find out what the knock is. The question is how much money will you spend to fix it. Could be lower end bearing, wrist pin bearing, piston slap, exhaust valve or several different things. Some would be relatively inexpensive repair, but if you wait for your engine to finally tell you for certain what it is, the price usually increases dramatically.
Have you let someone with a lot of mechanical experience listen to it??

 

[eagles22793]

I think I will buy a gasket kit and take the engine apart and repair it the right way. I use 93 octane from the gas station. Should I be using 106 I think it is for race fuel for dirt bikes or can I stick with the 93 octane?

 

[2-Strokes 4-ever]

When the engine's apart take the head and cylinder to your local shop and have them do some measuring... if some material was taken off the base of the cylinder or there were head mods, you'll have a better idea of fuel required.

I wouldn't even start the bike with the knock you're hearing. I was a BOZO when leading a group on our annual Dual Sport Series ride, heard a knock 1/2 way through the first day but just kept riding that day and the next day too. Had a main bearing cage coming apart... new mains, seals, rebuilt crank, rod, cylinder replate, ruined a top-end that only had about 5 hours on it. $$$.
If the main bearing would have gone out all together the damage could have been much worse... I was fortunate it didn't.

 

[eagles22793]

The knock has been in the bike since I got it and I was surprised nothing came apart in the top end when I went to the track the past weekend I put about 12 hours on it in two days at the track with the knock in it.

 

[eagles22793]

I just got the top end apart and found out that the piston is what is making the knocking sound but I know a one ring piston will make a little bit of noise but I am not sure how much. They do require more servicing then a stock two ring piston. Should I be fine by just putting a ring on the old piston and a gasket kit? The piston looks good as well as the cylinder.

 

[2strokerfun]

You really need to measure them to be sure. If you don't have a set of micrometers and bore gauges, take parts to good shop to measure. Make sure the cylinder is in round and within specs. But, how are you so sure this is what was causing the problem??

 

[eagles22793]

I am going to measure the cylinder to make sure but I dont know if this is causing the problem. I think I have enough experience to know that if everything thing is tight to where it is supposed to be and when I took off the top head and moved it back and fourth in the cylinder and it made a knocking sound like it sounded when running then I think this is most likley the problem. Would just putting a ring in be a good choice or should I replace everything?

 

[Rich Rohrich]

but I know a one ring piston will make a little bit of noise but I am not sure how much.

They don't make a knocking sound. If it's making that type of noise there is a problem.

Would just putting a ring in be a good choice or should I replace everything?

A good choice would to follow the advice you were given early and MEASURE IT. Regardless of how finely tuned you think your "eyecrometer" and "feelometer" are, they aren't good enough. Measure the piston and the bore for piston to wall clearance, out of round , and bore taper. Then check the upper rod bearing, the lower rod bearing and most important the mains.

It's a race bike not a weed whacker. :bang:

 

[rmc_olderthandirt]

I took off the top head and moved it back and fourth in the cylinder and it made a knocking sound

Describe to me how it moved to make this noise.

I have never experienced a piston moving during an inspection (at least not from an engine that was running when I tore it down) so I just can't fathom this.

Did it rock, fore and aft? Any significant movement there would indicate that the bore is too big for the piston.

If it rocked left/right then not only would the bore have to be too big but the wrist pin bearing would have to be shot.

If the knock occurs when you move the crank then I would think it was either the wrist pin or crank journal bearing.

The number of rings wouldn't have anything to do with it. My 125 had a single ring while the 250 had two.

Rod

 

[eagles22793]

The piston rocked back and forth front to back of the bike it did not have any play side to side. How do I check the main crank bearings with out spliting the engine? It looks that the cylinder has been honed out before which should not make the bore size any different. I will measure everything soon.

 

[eagles22793]

i took apart the bottom end and found pieces of bearing from one of the bearings on the govenor shaft beside the clutch. I noticed four balls fall and know where they go but why are they there I have never seen this before?