yz125 loss of compression after new rings

CheckpointGnarly
Edited Date/Time 10/24/2015 4:42am
couple weeks ago while i was out riding my bike lost all compression, took the top end apart and changed the rings and gaskets, put it back together and all was well, was starting up in 1-3 kicks every time for two ride days after that. today i was out, and my bike stalled, and when i went to kick it over it wouldn't start and it felt like there was hardly any compression in the kicker, so i threw it on the truck and took it apart to see if i could figure out what was wrong.


the head seems a little funky to me, not sure what's going on


the cylinder doesn't seem to have any noticeable scratches or anything on it anywhere


the piston has some visible scratches on the exhaust side



the ring seems like it's possibly warped?

any ideas what's goin on? it was riding awesome the last two times i was out, only had like maybe 1,5 hours on it with new rings installed
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ianhendry46
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ianhendry46 10/18/2015 7:09pm
10/18/2015 7:09pm
Did you measure the ring gap? Piston to cylinder clearance? Measure the cylinder(the bore), make sure all are within spec. If they are not, possibly send your cylinder off with a new piston and have them take care of ya. Good luck.
colintrax
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colintrax 10/18/2015 9:02pm
10/18/2015 9:02pm
I see scuffs in the cylinder. What oil ratio you running?
CheckpointGnarly 10/19/2015 1:59pm
10/19/2015 1:59pm
ive been running 32:1 all summer, was out the other day with some friends who have had 2 stroke yz's for many years and they said they've been running 50:1 forever... i was skeptical but decided to give it a try on sunday, rode for about 10 mins with 50:1 before she died.

i can run my finger around the cylender wall and cant feel any scratches at all,

the season was pretty much over anyways and i planned on a new piston and rings over the winter so i'm not terribly worried.

i'm pretty new to the 2 stroke world so just wanted to throw some pictures up and see what everyone out there thinks.
mb
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mb 10/19/2015 2:30pm
10/19/2015 2:30pm
The 50:1 is your answer.

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CheckpointGnarly 10/19/2015 2:36pm
10/19/2015 2:36pm
mb wrote:
The 50:1 is your answer.
That's what I thought too, just seems odd that I've got 2 or 3 buddy's running 50:1 in their yz' all year long with no proble a
project racer 10/19/2015 5:25pm
10/19/2015 5:25pm
Measure the bore diameter and check to see if it is in spec. Don't just throw pistons at it.

If the plating is still decent, you may be able to measure up a C or D size piston , if offered, to bring clearances back into spec.
CheckpointGnarly 10/19/2015 6:10pm
10/19/2015 6:10pm
i'll have to try and borrow a bore gauge from someone and double check on that, the cylinder is was just replaced by the last guy who owned it over last winter, probably only has about 30-35 hours on it since then
colintrax
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colintrax 10/19/2015 8:30pm
10/19/2015 8:30pm
I'm telling you I see scuffs in the wall. Take the jug to a shop and have them hone the cylinder and measure it.
Your buddies might get away with 50:1 but that's not enough oil for a 125. Run 32:1 or 40:1
I run my KTM at 40:1 if that makes a difference
JM485
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JM485 10/19/2015 10:26pm
10/19/2015 10:26pm
mb wrote:
The 50:1 is your answer.
CheckpointGnarly wrote:
That's what I thought too, just seems odd that I've got 2 or 3 buddy's running 50:1 in their yz' all year long with no proble...
That's what I thought too, just seems odd that I've got 2 or 3 buddy's running 50:1 in their yz' all year long with no proble a
50:1 is definitely not your answer, I ran that for years in my cr125 for trails and never had issues, I run it at 44:1 or 40:1 at the track now that I am faster but I still think I could get away with 50:. Was the bike warmed up well before being ridden and were the new rings broken in and seated before really pounding on it? If I had to guess the cylinder is out of spec from the first time it lost compression, and since the old piston was put back in there the cylinder to piston clearance is way too big. I can't tell you for sure what you answer is, but running 50:1 is not what killed your motor, that I can tell you.
Paw Paw 271
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Paw Paw 271 10/20/2015 6:49pm
10/20/2015 6:49pm
The cylinder shows a lot of blowby. The cylinder looks to be well worn and in need of service. The piston does not look good either.

Paw Paw
CheckpointGnarly 10/21/2015 1:25pm
10/21/2015 1:25pm
Paw Paw 271 wrote:
The cylinder shows a lot of blowby. The cylinder looks to be well worn and in need of service. The piston does not look good either...
The cylinder shows a lot of blowby. The cylinder looks to be well worn and in need of service. The piston does not look good either.

Paw Paw
yea the piston will be replaced for sure, i'm in the works of finding a shop that will check my cylinder and give it a quick hone for me. it was brand new about 30 hours ago so it shouldn't be too worn out. hopefully i wont have any more problems with it in the spring.

the whole bike is about to get a make over, so i'm not in a huge rush. i'll probably start up a build thread once i get some progress and that ball starts rollin
Phillip_Lamb
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Phillip_Lamb 10/21/2015 3:46pm
10/21/2015 3:46pm
mb wrote:
The 50:1 is your answer.
CheckpointGnarly wrote:
That's what I thought too, just seems odd that I've got 2 or 3 buddy's running 50:1 in their yz' all year long with no proble...
That's what I thought too, just seems odd that I've got 2 or 3 buddy's running 50:1 in their yz' all year long with no proble a
JM485 wrote:
50:1 is definitely not your answer, I ran that for years in my cr125 for trails and never had issues, I run it at 44:1 or...
50:1 is definitely not your answer, I ran that for years in my cr125 for trails and never had issues, I run it at 44:1 or 40:1 at the track now that I am faster but I still think I could get away with 50:. Was the bike warmed up well before being ridden and were the new rings broken in and seated before really pounding on it? If I had to guess the cylinder is out of spec from the first time it lost compression, and since the old piston was put back in there the cylinder to piston clearance is way too big. I can't tell you for sure what you answer is, but running 50:1 is not what killed your motor, that I can tell you.
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with less oil than usual you'll get more friction. but i would have a shop look it, it might need the honed
USMCMXer
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USMCMXer 10/23/2015 4:06pm
10/23/2015 4:06pm Edited Date/Time 10/23/2015 4:09pm
Phillip_Lamb wrote:
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with...
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with less oil than usual you'll get more friction. but i would have a shop look it, it might need the honed
This is going in a real bad direction. First, the 50:1 did not ruin your engine! Second, "jetting tuned on the lean side to compensate for more oil" is technically backwards, because more oil actually LEANS out the mixture as there is more oil and less fuel within the mixture. Lean and rich have to do with air/fuel ratio, therefore more oil makes it leaner (less fuel to air, as there is more oil). I know everyone wants to add oil to "richen" it up, but in terms of jetting, you are making it leaner. You are however putting more oil in the top-end, that part is true.

As for your problems, your cylinder is covered in blow-by, that is never good and indicates poor ring seal. Be honest with yourself if you want to really fix this: Do you let the cylinder get hot before revving it to the moon? If you cannot feel heat in the radiator, you should not be "cleaning it out" WFO, because that creates what is evident on the exhaust side of your piston; cold seizing. That soft, aluminum piston heats up a lot quicker than that hard, surrounded by coolant, steel lined cylinder. Therefore, if you don't let it warm up properly, you will get the marks you have on the exhaust side of the piston. This will always lead to compression loss.

Way forward: Make sure you get a new piston fitted to your cylinder and that your ring end-gap is set correctly, i.e. send the whole works to be re-plated or send the cylinder off and buy a piston from whomever replates (Millenium technologies, etc..) it. You'll wanna get everything fresh and not keep putting new pistons and rings into a worn cylinder.

That was a mouthful, I know. Also ensure you don't have any intake side air leaks or an air leak on your flywheel side crank seal. Both of these will create a lean condition. If your clutch side crank seal is blown, you'll get a lot of extra smoke and oil residue on the exhaust end cap.

Hope this helps a little bit. Good luck! I remember being super frustrated when I was a kid that my 87 CR kept doing this and I learned the expensive way to fix it right, vice trying to keep throwing cheap parts at it. I guess I learned if you cannot afford to fix it right, how many times can you afford to keep fixing it wrong?
colintrax
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colintrax 10/23/2015 4:16pm
10/23/2015 4:16pm
Phillip_Lamb wrote:
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with...
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with less oil than usual you'll get more friction. but i would have a shop look it, it might need the honed
USMCMXer wrote:
This is going in a real bad direction. First, the 50:1 did not ruin your engine! Second, "jetting tuned on the lean side to compensate for...
This is going in a real bad direction. First, the 50:1 did not ruin your engine! Second, "jetting tuned on the lean side to compensate for more oil" is technically backwards, because more oil actually LEANS out the mixture as there is more oil and less fuel within the mixture. Lean and rich have to do with air/fuel ratio, therefore more oil makes it leaner (less fuel to air, as there is more oil). I know everyone wants to add oil to "richen" it up, but in terms of jetting, you are making it leaner. You are however putting more oil in the top-end, that part is true.

As for your problems, your cylinder is covered in blow-by, that is never good and indicates poor ring seal. Be honest with yourself if you want to really fix this: Do you let the cylinder get hot before revving it to the moon? If you cannot feel heat in the radiator, you should not be "cleaning it out" WFO, because that creates what is evident on the exhaust side of your piston; cold seizing. That soft, aluminum piston heats up a lot quicker than that hard, surrounded by coolant, steel lined cylinder. Therefore, if you don't let it warm up properly, you will get the marks you have on the exhaust side of the piston. This will always lead to compression loss.

Way forward: Make sure you get a new piston fitted to your cylinder and that your ring end-gap is set correctly, i.e. send the whole works to be re-plated or send the cylinder off and buy a piston from whomever replates (Millenium technologies, etc..) it. You'll wanna get everything fresh and not keep putting new pistons and rings into a worn cylinder.

That was a mouthful, I know. Also ensure you don't have any intake side air leaks or an air leak on your flywheel side crank seal. Both of these will create a lean condition. If your clutch side crank seal is blown, you'll get a lot of extra smoke and oil residue on the exhaust end cap.

Hope this helps a little bit. Good luck! I remember being super frustrated when I was a kid that my 87 CR kept doing this and I learned the expensive way to fix it right, vice trying to keep throwing cheap parts at it. I guess I learned if you cannot afford to fix it right, how many times can you afford to keep fixing it wrong?
Just as an fyi, there's no steel liner. Aluminum piston and aluminum cylinder. Steel rings.
CheckpointGnarly 10/23/2015 4:40pm
10/23/2015 4:40pm
Phillip_Lamb wrote:
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with...
its possible this couldve contributed if the jetting is tuned on the lean side to compensate for the extra oil. if you run a gas with less oil than usual you'll get more friction. but i would have a shop look it, it might need the honed
USMCMXer wrote:
This is going in a real bad direction. First, the 50:1 did not ruin your engine! Second, "jetting tuned on the lean side to compensate for...
This is going in a real bad direction. First, the 50:1 did not ruin your engine! Second, "jetting tuned on the lean side to compensate for more oil" is technically backwards, because more oil actually LEANS out the mixture as there is more oil and less fuel within the mixture. Lean and rich have to do with air/fuel ratio, therefore more oil makes it leaner (less fuel to air, as there is more oil). I know everyone wants to add oil to "richen" it up, but in terms of jetting, you are making it leaner. You are however putting more oil in the top-end, that part is true.

As for your problems, your cylinder is covered in blow-by, that is never good and indicates poor ring seal. Be honest with yourself if you want to really fix this: Do you let the cylinder get hot before revving it to the moon? If you cannot feel heat in the radiator, you should not be "cleaning it out" WFO, because that creates what is evident on the exhaust side of your piston; cold seizing. That soft, aluminum piston heats up a lot quicker than that hard, surrounded by coolant, steel lined cylinder. Therefore, if you don't let it warm up properly, you will get the marks you have on the exhaust side of the piston. This will always lead to compression loss.

Way forward: Make sure you get a new piston fitted to your cylinder and that your ring end-gap is set correctly, i.e. send the whole works to be re-plated or send the cylinder off and buy a piston from whomever replates (Millenium technologies, etc..) it. You'll wanna get everything fresh and not keep putting new pistons and rings into a worn cylinder.

That was a mouthful, I know. Also ensure you don't have any intake side air leaks or an air leak on your flywheel side crank seal. Both of these will create a lean condition. If your clutch side crank seal is blown, you'll get a lot of extra smoke and oil residue on the exhaust end cap.

Hope this helps a little bit. Good luck! I remember being super frustrated when I was a kid that my 87 CR kept doing this and I learned the expensive way to fix it right, vice trying to keep throwing cheap parts at it. I guess I learned if you cannot afford to fix it right, how many times can you afford to keep fixing it wrong?
Thanks for the detailed post! Honestly I probably didn't give it as much time to warm every time I rode which makes sense after reading that, I'll definitely be more stringent in the warm up's next spring.

I've got everything apart now and taking it to a motor shop next week to get measured up, and honed and piston to match as long as there is too much damage.

Just for my own future reference what was it about the cylinder that looks like blow by? I've seen a lot of piston diagnostics pics out there but have had a hard time finding anything for cylinders
Paw Paw 271
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Paw Paw 271 10/24/2015 4:42am
10/24/2015 4:42am
Just look at all the discoloration below the ports. That area should be clean and not discolored. I also see where the piston has become scuffed at the lower skirts and thus the piston is rocking in the cylinder.
I think you will find that the cylinder will need more that a clean up and hone job.

Paw Paw

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