Posts
194
Joined
6/21/2017
Location
Bellfountain, OR, USA
Edited Date/Time
1/1/2026 1:58pm
Need some help diagnosing what happened here. Bike is a ‘23 yz125, just replaced piston/ring about 3 engine hours prior to this happening.
Original piston showed signs of detonation so i mixed up some higher octane fuel. Went out this past Sunday and was able to put in 4 solid moto’s until this happened. Just lost compression.
Upon tear down last night the cylinder looks to be mostly ok. The head is beat up a little and obviously the piston is damaged. The little metal piece fell out of the pipe.
My theory is the bike was running too lean and melted the electrode off the spark plug which pounded around. But then I am totally unsure that little metal piece was once the electrode.
Any ideas?
Check your lower rod bearing, it may be coming apart…
What were the weather conditions, what fuel, oil and ratio? Is that the correct plug heat range?
Rod seems fine, spins very smooth and minimal side to side play. Bike has a total of 30 hours on it. Weather conditions were colder than normal...40-45*F. Have been using Motorex cross power premix at 36:1 and a mix of 1 gal. 110 Renegade to 4 gals 97 non-ethanol. (first time using this gas) Plug is a BR9-EVX.
That needle on top of the piston is either from wrist pin bearing or lower rod bearing…
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It is not from the small end rod bearing. Maybe this is the perfect excuse to tear down and long rod the engine
Thanks for the insight.
Stock jetting?
No it is not, I've been playing around for awhile and just recently put in the Y4 needle.
Jetting as of right now:
168 main
70 pilot
y4 needles 3rd
70 powerjet
airscrew about 1.25 out
Interesting... You're not the first one here to have detonation or failure with the Y4 needle. That's also richer jetting than PC recommends so I would've assumed it's safe. But, could've been something else going on as well.
That metal piece is not the remnant of the electrode. If you have all of the needles in the top end bearing then you most likely had something introduced into the intake tract like a "rogue" carb float pin or needle from another needle bearing you prior replaced. Could have been stuck on a paper towel or rag and dropped into the air box by accident. If it wasn't for your statement that this happened after the "3 hours" you mentioned, I would have also considered something accidentally dropping into the bottom end while performing the top end job. I think you are right about the electrode being melted by the lean condition. A cool day, gas you don't normally use...I think that helped make you run lean. Good luck getting to the bottom of it!
I ran the same jetting as OP, but it was 60° out. Bike was boardline with the Y4 needle in position 3. Did one lap and came back to swap it to position 4. It’s hard to say because I was running T2 so it will favor richer jetting being an oxygenated fuel.
I was using the Y4 needle for a time and also moved it to clip 4. VPR fuel at 32:1. Thankfully no signs of detonation at teardown though.
Well i checked the lower rod bearing tonight. Was able to go to TDC and spin the bearing with a pick....all needles there. Next step is to removed float bowl and inspect. The rogue float pin was also suggested by a friend.
I didn't read every single word above, so you may have mentioned this already.
The wire diameter of your mystery object looks like a wrist-pin circlip for a YZ125 to me.
Do you have possession of both intact circlips from this damaged piston?
Could it be a circlip dropped in during the previous top-end replacement?
Can you account for both circlips from that previous top end?
It's entirely plausible to have a circlip bounce around in the bottom end for two or three hours until it is broken up enough to fit through a transfer port.
In my opinion, this is the most likely scenario.
At a minimum, you should do a very thorough job of checking that there aren't any fragments in the bottom end.
I hadn’t thought of that. I did account for both circlips from this piston, both still sitting on the work bench. Pretty sure the previous ones accounted for also but can’t be 100%.
I will add, that the foreign object is damn near identical to the wrist pin needles. Kinda squared up or at least had flat spots on it maybe from bouncing around. I’m planning on having a shop day tomorrow so will get the oil drained and look carefully.
Stuff like that happens sometimes.
At least it's a 125 and relatively inexpensive to repair.
It's good practice to have a brand-new, 100% clean rag standing by to pack in around the rod as soon as you lift the cylinder off the cases.
That prevents any crumbs or parts being dropped inside.
The transmission oil in a two stroke doesn't enter the crankcase area.
If there's anything left in in the crankcase, it won't be in the oil.
Most of the time on a 125, you'll have some "glitter" in the oil from the clutch and gearbox.
That's not a problem unless you see chunks.
Yeah I know all that....kinda delirious i guess from driving 7 hours today to ride 80 miles of prime desert.
How wild would it be if that was actually the ground strap from the plug, but hammered thin and elongated. Would that even be possible? I wouldn't initially think so, but who knows with enough heat. I would be curious to take LxWxH measurements and see if the volume matches what's missing from the plug
That was one of my first thoughts as well. Maybe I will try and take some volume measurements like you mentioned.
Right now I'm kinda on hold to tear apart further. I'm trying to decide to just toss another oem piston in there or say F it and have some cylinder porting done or go further with a big bore + porting.
Pit Row
FYI, here are some more pictures showing a wrist pin needle I pulled out for comparison as well as the head dome.
My gut reaction is that bearing needles aren't that malleable and wouldn't distort like that against aluminum, especially when you consider the missing material volume.
But, if it was stuck down in the bottom end for a while, the steel crank and rod could have made those edits to the needle before it migrated up into the top end.
Whatever it is and however it happened, you're pretty lucky the cases and cylinder aren't mangled.
You have some minor detonation damage around the edge of that bore and the mating edge of the head.
It is cheap insurance to run a mix of 110 leaded and 92 ethanol free pump fuel in all two strokes.
I run 50/50 mix in my YZs, but you can probably get away with a 1 gallon to 4 gallon mixture.
I was just able to source some race fuel recently so I just started running a mix of 3 gallons 97 to 1 gallon 110 octane.
Would you advise to clean up the detonation marks on the cylinder if I were just put in another OEM piston? Not sure if that could have damaged the nikasil plating.
That 3:1 mix will be great in the YZ125.
I wouldn't mess with the top edge of the cylinder.
The ring doesn't come up high enough to touch that edge, so the plating won't be at risk of peeling / flaking off.
Given that you had steel parts floating around in there, it's worth taking a close look with a bright light at all the port edges and the exhaust bridge. Also check that your power valves function smoothly, etc.
That stock head can be re-cut to clean it up. With a good spec, it will improve power, response and make jetting more consistent than the stock profile.
I re-cut the stock heads on my YZ125s in my own shop to tighten squish clearance to a target of 0.030in (a safe range is 0.031in to 0.027in).
Having that work done to the stock head makes the bike run more consistent and clean, but it does require the race fuel mix since there's a slight bump in compression.
There are plenty of aftermarket head options out there too, but there's nothing wrong with the stock castings.
Thanks for that info.
I do feel a bit better about just putting a new top end together. I just measured the volume of the foreign object at 18.00mm versus 20.38mm, so like 13%. So it sure looks plausible they the object IS the grounding electrode.
On a side note, would you expect straight 110 leaded (VP110) to work well in a YZ125 with reduced squish?
With corrected jetting, that fuel will run clean and you'll never have detonation issues.
In general, a bike jetted for pump gas will run fatter the higher the percentage of race fuel you run.
I think there's a whole thread on jetting the 2022+ YZ125s with the Keihin carburetor...
Straight race fuel isn't necessary in a relatively stock YZ125, even with a re-cut head.
Straight race fuel does provide a wider window of protection from heat and detonation.
With jetting and race fuel, the bike will not feel loaded up as easily and it will clean out quicker on quality fuel.
Lots of guys will point out that you'll lose a little response running excess octane beyond what the bike needs.
The true advantage of race fuel is that you can run a more aggressive engine tune...
Changes to parameters like compression ratio, ignition timing, port timing, and jetting are necessary to get the most out of the fuel.
The rider and terrain matter too. A pro on a fast, deep sand track demands more from the bike and will push it closer to detonation than a beginner on a slow, hard packed surface.
Thanks. I've actually been running VPR in it since new and recently moved to an area with VP110 at the pump for less than half the price. There's a lot of misinformation online about running the high octane, non-oxy fuels like C12 and 110 in stock bikes, but when it comes to VPR and T2 (oxy) those are widely recommended.
I've never done a back to back fuel comparison with all the different options out there and I've never tried VPR.
Something like VP110 will run a hell of a lot better than pump premium alone in most two strokes if you spend time on your jetting.
I have a local pro buddy who runs all his two strokes on 100% VP110.
He's younger and faster than I am and pretty savvy with jetting and testing to find a good setup.
His bikes always run very clean and strong with no issues.
I think the misinformation about some fuels comes from the fact that you can't just pour in a different fuel and expect it to run properly without any jetting changes.
On the other hand, I've compared Sunoco 110, Trick 110, and 100LL aviation gasoline. The Sunoco and 100LL give my YZ125 and YZ250 a laggy feeling with reduced throttle response and reduced torque. They both ran clean and are far better than straight pump premium, but the Trick gives a little better performance that I think is worth the extra cost.
Be aware that the Y4 needle is intended to be used with a #80 pilot and specific air screw.
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