Read my piston - YZ125

jridout12
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jridout12
Edited Date/Time 7/13/2022 12:55pm
Just did a top end at 30 hours. Had quite a bit of blow by so I’m going to be shortening my next top end interval. Getting mixed opinions on if the bike was running rich or lean. Thanks for any input


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ar15
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ar15 7/13/2022 11:17am
7/13/2022 11:17am
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a results also the top of the piston will get hotter. Due to the high temperature the oil in the fuel/air mixture will burn and deposit as carbon to the top, and also the bottom of the piston. Ideally the top edge of the piston should be clear of carbon (say, about the width if a finger nail).

I would bump up the main jet.

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jridout12
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jridout12 7/13/2022 12:09pm
7/13/2022 12:09pm
ar15 wrote:
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a...
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a results also the top of the piston will get hotter. Due to the high temperature the oil in the fuel/air mixture will burn and deposit as carbon to the top, and also the bottom of the piston. Ideally the top edge of the piston should be clear of carbon (say, about the width if a finger nail).

I would bump up the main jet.

Alright, stock main jet is a 430. I had a 420 in. I may just revert back to stock jetting and go from there
jridout12
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jridout12 7/13/2022 12:10pm
7/13/2022 12:10pm
ar15 wrote:
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a...
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a results also the top of the piston will get hotter. Due to the high temperature the oil in the fuel/air mixture will burn and deposit as carbon to the top, and also the bottom of the piston. Ideally the top edge of the piston should be clear of carbon (say, about the width if a finger nail).

I would bump up the main jet.

I am running leaded fuel though, could those possibly be lead deposits on top?
FGR01
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FGR01 7/13/2022 12:21pm
7/13/2022 12:21pm
As AR15 said, that bike had some combustion temps going on. Would be interested to know what exact fuel, oil, and ratio you are running. These also can contribute to the appearance of the carbon. The lead is what gives the carbon that slight grey color.

By the looks of that blowby, I assume you were running it like that with no ring for about the last 10 hrs? Hahah I keed Laughing

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ar15
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ar15 7/13/2022 12:27pm
7/13/2022 12:27pm
ar15 wrote:
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a...
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a results also the top of the piston will get hotter. Due to the high temperature the oil in the fuel/air mixture will burn and deposit as carbon to the top, and also the bottom of the piston. Ideally the top edge of the piston should be clear of carbon (say, about the width if a finger nail).

I would bump up the main jet.

jridout12 wrote:
I am running leaded fuel though, could those possibly be lead deposits on top?
I have no experience with leaded fuel, but I would not expect it to give a deposit like this. It looks like carbon (= burned oil). If it were a deposit from the lead, I would expect it to have a different color.

What type of tracks do you ride? If you ride (deep) sand tracks you will require a much more richer fuel/air ratio. In our case ( KTM SX250), we needed to go three sizes higher on the main jet than the user manual prescribes for sand tracks.
jridout12
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jridout12 7/13/2022 12:45pm
7/13/2022 12:45pm
ar15 wrote:
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a...
It's running (much) too lean. The effect of a lean mixture is that the temperature during the combustion will be higher than normal, and as a results also the top of the piston will get hotter. Due to the high temperature the oil in the fuel/air mixture will burn and deposit as carbon to the top, and also the bottom of the piston. Ideally the top edge of the piston should be clear of carbon (say, about the width if a finger nail).

I would bump up the main jet.

jridout12 wrote:
I am running leaded fuel though, could those possibly be lead deposits on top?
ar15 wrote:
I have no experience with leaded fuel, but I would not expect it to give a deposit like this. It looks like carbon (= burned oil)...
I have no experience with leaded fuel, but I would not expect it to give a deposit like this. It looks like carbon (= burned oil). If it were a deposit from the lead, I would expect it to have a different color.

What type of tracks do you ride? If you ride (deep) sand tracks you will require a much more richer fuel/air ratio. In our case ( KTM SX250), we needed to go three sizes higher on the main jet than the user manual prescribes for sand tracks.
I do ride a lot of sand, it's not super deep but it is sandy. The last time I rode the bike it was 90 degrees and humid so that's why I thought I may be too rich. But I think I will just go back to stock and make slight adjustments from there
jridout12
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jridout12 7/13/2022 12:48pm
7/13/2022 12:48pm
FGR01 wrote:
As AR15 said, that bike had some combustion temps going on. Would be interested to know what exact fuel, oil, and ratio you are running. These...
As AR15 said, that bike had some combustion temps going on. Would be interested to know what exact fuel, oil, and ratio you are running. These also can contribute to the appearance of the carbon. The lead is what gives the carbon that slight grey color.

By the looks of that blowby, I assume you were running it like that with no ring for about the last 10 hrs? Hahah I keed Laughing
Haha no I just took the ring off before this pic but I am going to shorten my top end intervals for next time. I run 50/50 pump 91 and sunoco 110 race fuel mixed 32:1 with klotz super techniplate. The cylinder looked good and everything had a nice coat of oil.
ar15
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ar15 7/13/2022 12:55pm
7/13/2022 12:55pm
Your piston is the result of 30 hours of riding, not just from the last time. When you have replaced the piston, clean the carb, put in a new spark plug and try to do a plug chop test to see if you're running lean or rich.

BTW, when we were riding 125 we replaced the piston after 20 hours max.
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wiedeman
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wiedeman 7/14/2022 8:04pm
7/14/2022 8:04pm
1 size less than stock main.... In sand???? You are asking for trouble. On my 22 yz 125, I am up 5 sizes on the main and 1 clip richer for sand riding.

You are obviously lean. Any more than a dime size spec of carbon on the bottom of a piston is a sure indicator of 'running on the fine edge'. On my 09 yz 125, I would normally run a 460 in deep sand.

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