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Reading a 4 stroke piston

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Hi, I tore down the top end to inspect and to replace a leaky head gasket. Everything seems good up there, no concerns over cylinder or piston wear and the cams and valve gaps are all good.

I have a question though on "reading" a piston.

Right between the intake valve cut aways on the piston, there's a very sparsely fouled area compared to the rest of the piston. I have done top end rebuilds before, but this seems a little different from anything I've seen before.

Is there any assumption to be made (ie. jetting too lean) or does this seem perfectly normal?

Constructive feedback appreciated.

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I'm sure somewhere here would ask to see a picture

 Of course they would :p  

I posted from my phone, attached photo but it obviously didn't upload.

'Ere it is now, I hope.

IMG_20150815_142644.jpg

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Hi, I tore down the top end to inspect and to replace a leaky head gasket. Everything seems good up there, no concerns over cylinder or piston wear and the cams and valve gaps are all good. 

I have a question though on "reading" a piston. 

Right between the intake valve cut aways on the piston, there's a very sparsely fouled area compared to the rest of the piston. I have done top end rebuilds before, but this seems a little different from anything I've seen before. 
Is there any assumption to be made (ie. jetting too lean) or does this seem perfectly normal? 

Constructive feedback appreciated. 

 

EDIT: I put this here as I believe this is jetting related. 

IMG_20150815_142644.jpg

Edited by genR8r

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that isnt anything, just where the intake charge is washing it i presume .. one good long stretch winding it out and then inspecting would probably change the way the top looks .. but on the top you just generally dont want to see damage like obvious errosion or dings .. the sides of the skirt would be more telling to the over-all condition of the piston ..

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that isnt anything, just where the intake charge is washing it i presume .. one good long stretch winding it out and then inspecting would probably change the way the top looks .. but on the top you just generally dont want to see damage like obvious errosion or dings .. the sides of the skirt would be more telling to the over-all condition of the piston ..

 

And interesting that the washing of the top of the piston is more on the right intake. I presume because the int. manifold comes in from the left side, making a slight bend into the cyl. so the heavier particals (fuel) tend to go more through the right port. And i guess the same reason if dirt and dust get sucked through the filter, that the right int. valve gets worn and is the first to need reshimming.

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That piston looks OLD, and lots of PITS in it......

 

If that is the intake side of the piston, that is normal.

 

The motor looks to be very rich from af ratio or from passion oil.

 

What does the piston / ring area look like.....I'll bet there is blow by.....

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That piston looks OLD, and lots of PITS in it......

 

If that is the intake side of the piston, that is normal.

 

The motor looks to be very rich from af ratio or from passion oil.

 

What does the piston / ring area look like.....I'll bet there is blow by.....

Most of that surface "pitting" came off with Q20 and a ra :thumbsup:

The piston and rings are in great shape, the skirt has no chipping, and compression is good.

What could be throwing you off, and  maybe you missed that line, but I went in after a leaky head gasket, maybe that has to do with some of the deposits.

Passion oil you say?! I keep that in my side stand, not in my bike  :p

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that isnt anything, just where the intake charge is washing it i presume .. one good long stretch winding it out and then inspecting would probably change the way the top looks .. but on the top you just generally dont want to see damage like obvious errosion or dings .. the sides of the skirt would be more telling to the over-all condition of the piston ..

Thanks, the skirt and surface which was inspected after cleaning with Q20 and a rag, is great shape, no dings, pitting or cracks.

My post wasn't because I"m concerned about the piston, I was hoping it would give an indication of my jetting, thinking that a rich mixture would wash the intake surface clean. I am comparing this to previous experience on some other bikes. the difference between the intake and exhaust side has never seemed this stark.

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And interesting that the washing of the top of the piston is more on the right intake. I presume because the int. manifold comes in from the left side, making a slight bend into the cyl. so the heavier particals (fuel) tend to go more through the right port. And i guess the same reason if dirt and dust get sucked through the filter, that the right int. valve gets worn and is the first to need reshimming.

Interesting observation and hypothesis  :thinking:  

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