Yz 125 jet problem (now seized piston)

Hrzk75
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1969th
7/21/2020 9:41am Edited Date/Time 7/29/2020 12:36am
Hi, i tried to Google but could not find info on the standard jetting.

Does the 2015+ models have a different jetting from the earlier ones?

I installed the new airbox(15+) on my 2013 and it wont idle anymore so im just wondering if the new airbox have more airflow or something and need bigger jetts. I dont know Much about this since i have never needed to change anything before in a carb
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dfr136
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7/21/2020 12:20pm
2011 and up has 430 main 40 pilot 43-74 needle, thats for the us model.
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Hrzk75
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7/22/2020 10:49am
Thx for the answers, 30 min past this post the bike seized, cylinder and piston is Done.

When i Tore down the top end i checked the spark plug and it was like burnt but white, no dark colours on it so i suppose it has run lean? Or does it not work to check sparkplug colour 2-3 hours past run time?

Its weird since it has run 13 hours without a problem with new piston, 40 hours since i bought it(122 total)

Anyway the main jet was 430 and pilot 40. Needle clip on 3rd, i have sent away the cylinder for a replacement but i wanna know why this happened, i lowered the clip 1 position and have ordered a 440 main and 42,5 pilot wich i Will try out. Is more then 3% oil mixture needed on a 125?
Hrzk75
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7/22/2020 10:57am




The piston took a beating and lost some of the material that holding circlip into place, have not found any trace of Either the broken piston parts or the circlip, how high chance is it that they have gone out into the Exhaust? Or is it Greater chance of the parts hiding under the crank?

Also found some chrome material has come off from the slide, not sure if that is new or if it has been there for some time, can it cause any troubles?

The Shop

dkurtd
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7/22/2020 11:08am
Although I can't see the rest of the piston, if I was betting the circlip came out and did the damage you see. I would need to see more of the piston to call it a lean seizure.
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Hrzk75
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7/22/2020 11:34am
dkurtd wrote:
Although I can't see the rest of the piston, if I was betting the circlip came out and did the damage you see. I would need...
Although I can't see the rest of the piston, if I was betting the circlip came out and did the damage you see. I would need to see more of the piston to call it a lean seizure.
i dont have the piston close right now but can post more pictures tomorow of it, but it does have damage on other side aswell.

what do you think about the slide? is it safe you use it more? and would you split the engine to look for the clip?
dkurtd
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7/22/2020 11:57am Edited Date/Time 7/22/2020 11:58am
Usually a lean seizure will show scoring along the exhaust port side, typically. As for splitting the cases, I would recommend it especially if I couldn't find the circlip and you say it has 122 hours on it. You almost surely have piston and clip material hiding in the main bearings.
Hrzk75
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7/22/2020 12:36pm
dkurtd wrote:
Usually a lean seizure will show scoring along the exhaust port side, typically. As for splitting the cases, I would recommend it especially if I couldn't...
Usually a lean seizure will show scoring along the exhaust port side, typically. As for splitting the cases, I would recommend it especially if I couldn't find the circlip and you say it has 122 hours on it. You almost surely have piston and clip material hiding in the main bearings.
crank was replaced at 80 hours, but the safest thing to do would prob be spliting i guess.. nothing i do by myself (no tools for it) tried to blow some air under the crank and use a magnet pen but nothing, would feel better to find the clip. im not sure it creates so much preasure that the engine can blow out the clip from the exhaust if its not running, by just rolling with gear in, right? bike died in the air and after i landed i rolled for a few meters with gear in.
Paw Paw
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7/22/2020 4:38pm
You need to do a complete tear down as the piston damage looks to be from something other than jetting issues. I suspect crank / rob bearing failure.

Paw Paw
FGR01
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7/22/2020 5:03pm
If the failure was caused by leanness, it wasn't from that jetting, unless you are riding in sub-freezing temps. The only thing that would make it lean enough to do that is a massive air leak somewhere. And even then, an air leak of that magnitude would have caused serious bogging that would have made you stop sooner.
DynoDan22
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7/22/2020 6:15pm
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of the wristpin boss has broken off of the piston. This makes me think the circlip popped out as the piston was traveling downward. The circlip got caught in the transferport and it broke the pin boss top of the piston. What brand piston was that?
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jaguar57
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7/22/2020 9:34pm
32:1 is a typical ratio for a semi-synthetic engine oil. It is too much oil for a full synthetic and not enough for a totally non-synthetic oil. Which one did you use?
If you plug was whitish and your jetting is typical and no trash got in the main jet to partially block it then you need to check for an air leak at the crank seals, head, or reed valve and intake boot. (Install a gas filter if it doesn't have one)
Make sure you have the mechanic check for up/down play at the big end bearing of the con rod so you'll know to replace that bearing if there is play.
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Hrzk75
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7/23/2020 4:29am
DynoDan22 wrote:
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of...
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of the wristpin boss has broken off of the piston. This makes me think the circlip popped out as the piston was traveling downward. The circlip got caught in the transferport and it broke the pin boss top of the piston. What brand piston was that?
Piston is a Prox, about 12 hours on it, was wery picky on making sure the circlips were in place, but yeah that is one possibilty, would a piston failure like that damage the other side of the piston aswell?
Hrzk75
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7/23/2020 4:31am Edited Date/Time 7/23/2020 5:24am
yeah sounds weird that it seized from jetts,since it prob have had the same jetting for atleast 80 hours, even during cold weather.

Could the damage on the slide i posted a picture on make it running leaner? otherwise it wery well can be the crank seal
*edit* would not an air leak at the crank seal make the idle rise?
Matt Fisher
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7/23/2020 10:37am
DynoDan22 wrote:
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of...
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of the wristpin boss has broken off of the piston. This makes me think the circlip popped out as the piston was traveling downward. The circlip got caught in the transferport and it broke the pin boss top of the piston. What brand piston was that?
Hrzk75 wrote:
Piston is a Prox, about 12 hours on it, was wery picky on making sure the circlips were in place, but yeah that is one possibilty...
Piston is a Prox, about 12 hours on it, was wery picky on making sure the circlips were in place, but yeah that is one possibilty, would a piston failure like that damage the other side of the piston aswell?
The circlip could certainly do damage to one side of the cylinder/piston, then jump to the other side and jack things up there as well. After that, what's left could then drop down into the big end of the rod and seize it up. Basically errant metal can really screw up your entire engine.

Keep in mind the offending metal might not be from a circlip. It could be a broken ring, ring pin, a reed screw, etc, etc.
Hrzk75
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1969th
7/23/2020 2:27pm Edited Date/Time 7/23/2020 2:28pm
DynoDan22 wrote:
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of...
That piston failure looks like the result of a circlip coming out not a lean seizure. If you look at your first pic, the top of the wristpin boss has broken off of the piston. This makes me think the circlip popped out as the piston was traveling downward. The circlip got caught in the transferport and it broke the pin boss top of the piston. What brand piston was that?
Hrzk75 wrote:
Piston is a Prox, about 12 hours on it, was wery picky on making sure the circlips were in place, but yeah that is one possibilty...
Piston is a Prox, about 12 hours on it, was wery picky on making sure the circlips were in place, but yeah that is one possibilty, would a piston failure like that damage the other side of the piston aswell?
The circlip could certainly do damage to one side of the cylinder/piston, then jump to the other side and jack things up there as well. After...
The circlip could certainly do damage to one side of the cylinder/piston, then jump to the other side and jack things up there as well. After that, what's left could then drop down into the big end of the rod and seize it up. Basically errant metal can really screw up your entire engine.

Keep in mind the offending metal might not be from a circlip. It could be a broken ring, ring pin, a reed screw, etc, etc.
yeah i just wanna make sure the cause of it, i hope it was my mistake and the clip actually came out from bad install, im not holding it wide open enough around the track to make it blow by the revs.

Anyway engine is out and will be tore down, wont take the chance of the clip and everything else is out from crank area.

anyone know if a lean pilot yet could have done some damage or do they generally seize from too small main jett? the euro standard is a 45 pilot and i have a 40 in right now.
Matt Fisher
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7/23/2020 6:55pm
Is that a Namura piston? I made the mistake of using that garbage and had a circlip go bad on me. Damage looked similar.

You can seize it from too small of a pilot, but usually that's accompanied by a long ride down a steep hill with the throttle chopped and the rpm's up high. Engine is at 8,000 rpm and only being fed by a circuit best equipped for idle-2,000 rpms for a long period of time.
Hrzk75
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7/24/2020 7:11am
Is that a Namura piston? I made the mistake of using that garbage and had a circlip go bad on me. Damage looked similar. You can...
Is that a Namura piston? I made the mistake of using that garbage and had a circlip go bad on me. Damage looked similar.

You can seize it from too small of a pilot, but usually that's accompanied by a long ride down a steep hill with the throttle chopped and the rpm's up high. Engine is at 8,000 rpm and only being fed by a circuit best equipped for idle-2,000 rpms for a long period of time.
no its a Prox piston, but i think this is from a bad quality of the piston aswell.
Spudnut
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7/26/2020 5:53am
ProX pistons are about as good as a door stop imo
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mattyhamz2
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7/26/2020 7:43pm
Spudnut wrote:
ProX pistons are about as good as a door stop imo
ProX makes the OEM pistons for the Japanese manufacturers.
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jaguar57
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7/29/2020 12:36am
opinions are like assholes, everyone has one
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