Thoughts on these pistons, YZ125

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2/19/2021 8:05 AM

Morning Vital Crew, wanted to get some thoughts on comparison of the two pistons out of my 2003 YZ125. Both pistons have the same approx 15 hours of use, both same Pro-x part.
Piston # 1 was ran With Klotz R-50 and VPR leaded and oxygenated fuel.
Piston # 2 was ran with Yamalube 2-R and VP110 leaded fuel. Both ran fantastic. Piston #1s ring was really stuck in the ring groove. Piston #2’s ring is free moving.
I jet accordingly for temp and track- I.e. richer for sand, ...but I’m guessing I’m still a little lean based on how hot these both have looked to have gotten? Photo
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2/19/2021 8:07 AM

Here is the head after piston #2.

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2/19/2021 8:50 AM

#1 is lean, for sure. You may want less oil in the premix if you are using that oil, or run a richer main jet. How's your splooge level? (Powervalve, silencer packing, etc.) I'm guessing "none."

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Braaapin' aint easy.

2/19/2021 8:52 AM

How many hours are on these things? They look pretty roached to me. I don't like the way the head looks or the top of piston #2 in comparison to how mine look during replacement.

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2/19/2021 9:09 AM

Falcon wrote:

#1 is lean, for sure. You may want less oil in the premix if you are using that oil, or run a richer main jet. How's your splooge level? (Powervalve, silencer packing, etc.) I'm guessing "none."

Thanks for the reply’s, No splooge for either, powervalve well lubed on both, much more so with 2-R. Actually was a giant puddle with 2-R.
The head was cleaner on piston #1 tho, which I find interesting.
I’ve went away from oxygenated fuel on the #2 piston, and I agree, you can tell for sure by the difference in excessively lean top of piston condition.
I think I’m still too lean on the main.

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2/19/2021 9:12 AM

Moto520 wrote:

How many hours are on these things? They look pretty roached to me. I don't like the way the head looks or the top of piston #2 in comparison to how mine look during replacement.

15 hours on each. What oil and ratios are you running? The majority of my riding and racing is on deeper tracks, think sand and Red Bud, tracks like that.

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2/19/2021 1:33 PM
Edited Date/Time: 2/19/2021 1:34 PM

Moto520 wrote:

How many hours are on these things? They look pretty roached to me. I don't like the way the head looks or the top of piston #2 in comparison to how mine look during replacement.

JB500 wrote:

15 hours on each. What oil and ratios are you running? The majority of my riding and racing is on deeper tracks, think sand and Red Bud, tracks like that.

You're hard/those conditions are tough on the bikes considering only 15hrs. You're definitely lean and looking for some leaded gas and possibly a new profile on your cylinder head that's more suited to WOT. #2 is definitely better than #1, that shit is about to grenade.

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KTM250SX YZ125 YZ85 PW80 ZX6 FZX700

2/19/2021 1:52 PM

Moto520 wrote:

How many hours are on these things? They look pretty roached to me. I don't like the way the head looks or the top of piston #2 in comparison to how mine look during replacement.

JB500 wrote:

15 hours on each. What oil and ratios are you running? The majority of my riding and racing is on deeper tracks, think sand and Red Bud, tracks like that.

YZed250 wrote:

You're hard/those conditions are tough on the bikes considering only 15hrs. You're definitely lean and looking for some leaded gas and possibly a new profile on your cylinder head that's more suited to WOT. #2 is definitely better than #1, that shit is about to grenade.

Yea, I ride this bike pretty good. It’s so much fun I’ve actually made it my primary bike, and I’m a 36 year old vet racer. I absolutely love it. As far as leader fuel, yes, that is all I run. The standard VP 110 on piston 2 looks better than the oxygenated from piston #1. I’m generally 1 to 2 up on the main, especially in sand, but obviously still lean. I’ll riches up across the board. Any other suggestions for oil? I like to keep it consistent across each 15 hour piston interval. I was thinking Supertechniplate or 927. Although the 2-R seems to have done a decent job as well. Just curious if something with castor may help protect a bit more, as this bike sees a lot of WOT.
Thanks

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2/19/2021 2:05 PM
Edited Date/Time: 2/19/2021 2:06 PM

JB500 wrote:

15 hours on each. What oil and ratios are you running? The majority of my riding and racing is on deeper tracks, think sand and Red Bud, tracks like that.

YZed250 wrote:

You're hard/those conditions are tough on the bikes considering only 15hrs. You're definitely lean and looking for some leaded gas and possibly a new profile on your cylinder head that's more suited to WOT. #2 is definitely better than #1, that shit is about to grenade.

JB500 wrote:

Yea, I ride this bike pretty good. It’s so much fun I’ve actually made it my primary bike, and I’m a 36 year old vet racer. I absolutely love it. As far as leader fuel, yes, that is all I run. The standard VP 110 on piston 2 looks better than the oxygenated from piston #1. I’m generally 1 to 2 up on the main, especially in sand, but obviously still lean. I’ll riches up across the board. Any other suggestions for oil? I like to keep it consistent across each 15 hour piston interval. I was thinking Supertechniplate or 927. Although the 2-R seems to have done a decent job as well. Just curious if something with castor may help protect a bit more, as this bike sees a lot of WOT.
Thanks

I'd run a hotter 10-series plug (like NGK B10EGV).

I'd definitely stick with a quality oil (such as 2-R or an ester-based synthetic or castor). I don't have a strong opinion on which oil brand. Look at your fuel: oil ratio. You'll see that the oil makes 1/32nd of the mix, or so.

Focus more on the gas and they don't come much better than VP110, and definitely the jetting. In my experience w the Mikuni, there is a fair amount of overlap on the main to the small circuits, so I'd work through the drain plug to tune the main only and you'll probably find that the other circuits are slightly fatter too, without any adjustments.

Things will naturally tend richer as the weather starts to warm up.

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KTM250SX YZ125 YZ85 PW80 ZX6 FZX700

2/19/2021 2:15 PM
Edited Date/Time: 2/19/2021 2:18 PM

You need to check your squish as all signs on the piston and head indicate it is not correct. Both pistons also show a lot of blow by and too much blow by for 15 hours. The second piston shows that it was getting less lubricant at the exhaust bridge holes in the piston at the bridge position as you can see that it was heating up more that the first piston.
Have your bore checked as it could be worn and causing a poor fit and thus excess blow by .
Did you check your ring end gap when installing them?

Paw Paw

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2/19/2021 2:39 PM

Paw Paw 271 wrote:

You need to check your squish as all signs on the piston and head indicate it is not correct. Both pistons also show a lot of blow by and too much blow by for 15 hours. The second piston shows that it was getting less lubricant at the exhaust bridge holes in the piston at the bridge position as you can see that it was heating up more that the first piston.
Have your bore checked as it could be worn and causing a poor fit and thus excess blow by .
Did you check your ring end gap when installing them?

Paw Paw

Thank you for the reply’s. Ring gap was checked and was in spec, and cylinder has only had these two pistons in it. New OEM B cylinder with a Pro x B piston. I will check squish before finishing this rebuild and getting the engine back in the frame. Thank you.

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2/19/2021 3:08 PM

Falcon wrote:

#1 is lean, for sure. You may want less oil in the premix if you are using that oil, or run a richer main jet. How's your splooge level? (Powervalve, silencer packing, etc.) I'm guessing "none."

JB500 wrote:

Thanks for the reply’s, No splooge for either, powervalve well lubed on both, much more so with 2-R. Actually was a giant puddle with 2-R.
The head was cleaner on piston #1 tho, which I find interesting.
I’ve went away from oxygenated fuel on the #2 piston, and I agree, you can tell for sure by the difference in excessively lean top of piston condition.
I think I’m still too lean on the main.

I notice more smoke and residue from 2R in my YZ250, also. I just switched over to Super-M to see how that works. If not, I'm just going to ante up and buy the Bel-Ray H1R that I started with on this bike. That shiz is expensive, but man, it works!

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Braaapin' aint easy.

2/19/2021 3:33 PM

What’s your break in process, that’s a lot of blow by

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2/19/2021 3:50 PM

soggy wrote:

What’s your break in process, that’s a lot of blow by

I very lightly oil the piston with mix oil, and upon running the bike, I’ll let it warm up 2-3 times for a couple minutes, do a few field rides through the gears, being sure to load it up, then ride as normal. I’m surprised by the blow by as well with this process, as it’s always seemed to seat ring(s) well.

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2/21/2021 5:39 AM

Skip the 2-3 warm ups on the stand and just ride it.

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2/21/2021 5:50 AM

soggy wrote:

Skip the 2-3 warm ups on the stand and just ride it.

For sure, I will do that. In 15 more hours when I pull this one we’ll see how this one looks. I’m also going to start taking a baseline and regular compression readings. Something I haven’t been doing.

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2/22/2021 9:17 AM

Both of those are way lean and you got lucky I'd say. Full carbon coverage over the entire top of the pistons due to high temps. Ideally you want to be rich enough to have some piston wash so that the carbon does not cover the entire top.

What do the undersides of the domes look like? I expect very large dark spots from the high temps.

I personally stay away from Klotz, 927, or any castor. I know lots of guys love it but my experience is excessive carbon in the PV and muffler. You can't go wrong with Super-M or K2. I run Super-M in all my 2-strokes and I run my 125's pretty hard. No issues at all.

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Powerband in every gear !

2/22/2021 10:20 AM

FGR01 wrote:

Both of those are way lean and you got lucky I'd say. Full carbon coverage over the entire top of the pistons due to high temps. Ideally you want to be rich enough to have some piston wash so that the carbon does not cover the entire top.

What do the undersides of the domes look like? I expect very large dark spots from the high temps.

I personally stay away from Klotz, 927, or any castor. I know lots of guys love it but my experience is excessive carbon in the PV and muffler. You can't go wrong with Super-M or K2. I run Super-M in all my 2-strokes and I run my 125's pretty hard. No issues at all.

Yes, under domes are dark, dark brown, very hot for sure. I’m going to riches up across the board for starters this season. I was leaning towards a castor type oil for any added protection it may offer, and seeing as I replace pistons and clean PV at 15 hours, I wasn’t concerned with the deposit as much. I will look into the K2 and Super -M as well. But like you said, too lean for sure. I just find it strange, as I’m generally running a 40 pilot, 430 main and needle on 4th clip position. Bike has been running fantastic other than a SLIGHT bog from zero throttle at first blip...but perhaps as I’m at WOT so often, the two sizes up on the main are still not enough. At least that’s what the pistons are saying. 35 minute motos in sand being lean certainly hasn’t helped.

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2/22/2021 2:18 PM
Edited Date/Time: 2/22/2021 2:20 PM

Moto520 wrote:

How many hours are on these things? They look pretty roached to me. I don't like the way the head looks or the top of piston #2 in comparison to how mine look during replacement.

JB500 wrote:

15 hours on each. What oil and ratios are you running? The majority of my riding and racing is on deeper tracks, think sand and Red Bud, tracks like that.

I also ride at Red Bud and LTMX etc. I use Yamalube R at 32:1. I have an FMF Pipe/ silencer and the Moto Tasinari. Other than that....she's stock. I do ride the piss out of that poor bike. I went one main jet fatter than stock...it rides better like this. I use pump gas. Those pistons (to me) look like they have been run on the ragged edge. I would definitely make some adjustments!

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2/22/2021 3:56 PM

FGR01 wrote:

Both of those are way lean and you got lucky I'd say. Full carbon coverage over the entire top of the pistons due to high temps. Ideally you want to be rich enough to have some piston wash so that the carbon does not cover the entire top.

What do the undersides of the domes look like? I expect very large dark spots from the high temps.

I personally stay away from Klotz, 927, or any castor. I know lots of guys love it but my experience is excessive carbon in the PV and muffler. You can't go wrong with Super-M or K2. I run Super-M in all my 2-strokes and I run my 125's pretty hard. No issues at all.

JB500 wrote:

Yes, under domes are dark, dark brown, very hot for sure. I’m going to riches up across the board for starters this season. I was leaning towards a castor type oil for any added protection it may offer, and seeing as I replace pistons and clean PV at 15 hours, I wasn’t concerned with the deposit as much. I will look into the K2 and Super -M as well. But like you said, too lean for sure. I just find it strange, as I’m generally running a 40 pilot, 430 main and needle on 4th clip position. Bike has been running fantastic other than a SLIGHT bog from zero throttle at first blip...but perhaps as I’m at WOT so often, the two sizes up on the main are still not enough. At least that’s what the pistons are saying. 35 minute motos in sand being lean certainly hasn’t helped.

Is your main seal on the ignition side is good shape? A bog at low throttle opening is more likely to be your pilot jet then the main.

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2/22/2021 4:17 PM

FGR01 wrote:

Both of those are way lean and you got lucky I'd say. Full carbon coverage over the entire top of the pistons due to high temps. Ideally you want to be rich enough to have some piston wash so that the carbon does not cover the entire top.

What do the undersides of the domes look like? I expect very large dark spots from the high temps.

I personally stay away from Klotz, 927, or any castor. I know lots of guys love it but my experience is excessive carbon in the PV and muffler. You can't go wrong with Super-M or K2. I run Super-M in all my 2-strokes and I run my 125's pretty hard. No issues at all.

JB500 wrote:

Yes, under domes are dark, dark brown, very hot for sure. I’m going to riches up across the board for starters this season. I was leaning towards a castor type oil for any added protection it may offer, and seeing as I replace pistons and clean PV at 15 hours, I wasn’t concerned with the deposit as much. I will look into the K2 and Super -M as well. But like you said, too lean for sure. I just find it strange, as I’m generally running a 40 pilot, 430 main and needle on 4th clip position. Bike has been running fantastic other than a SLIGHT bog from zero throttle at first blip...but perhaps as I’m at WOT so often, the two sizes up on the main are still not enough. At least that’s what the pistons are saying. 35 minute motos in sand being lean certainly hasn’t helped.

soggy wrote:

Is your main seal on the ignition side is good shape? A bog at low throttle opening is more likely to be your pilot jet then the main.

I’ve actually had a 37.5 pilot in....40 is stock, and I think I went the wrong way on the pilot...I have several larger ones on the way, along with larger mains. I was also thinking crank seal(s) but the bike purrs and idles so well, and is responsive to air screw adjustments. Thoughts on just replacing crank seals?

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2/22/2021 6:05 PM

I feel like these bikes have a hair more power when they are slightly rich vs slightly lean.

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2/22/2021 6:59 PM

Moto520 wrote:

I feel like these bikes have a hair more power when they are slightly rich vs slightly lean.

I plan to find that out, that’s for sure. And as soggy mentioned about ignition side crank seal, I’m going to go ahead and replace that as well. I got all excited remembering I had a new OEM dry side crank seal from my old 2000 YZ. Turns out the 2001 and up changed part numbers, so I just had to order a new one.

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