Piston blow by

[lllDennis]

So I've had this DT125r for 5 years now.
I've noticed a loss of power, checked in the exhaust port and noticed a lot of blowby, so had the top end rebuild september 2018 included rebore and hone. Was a joy to ride again (power wheelie in 1'st like it used to do).

Since then I only put 3000km's on it but when cleaning my exhaust I checked the port again and noticed a lot of blowby again. It was/is also down on power, not hitting the powerband like it used to do.

I want it running perfect again but don't want to end up with another rebuild in 3000km's.
Can anyone share some ideas/tips? I had it done at a Yamaha dealership.

Pictures (imgur.com)

[SpookDog]

What do you mean by blow by? Black oily stain on the piston below the rings, or oily crud in the exhaust manifold?
If it’s stain below the piston rings you’ll have to replace them or more...

[SpookDog]

Sorry, I meant excessive. 2strokes normally have some staining at the exhaust port don’t they? Due to the back pressure from the exhaust?
I usually think of blow by in 4 strokes, when you get gases chugging out the breather pipe caus the crankcase is over pressurised...

[lllDennis]

I mean the stains on the piston in the pictures.
This is not normal i think? I've done some rebuilds for my friends' bikes but none of them had staining like this and had a lot more km's

[Calum]

As @SpookDog says you are more talking four stroke really. There doesn't look anything abnormal there

[SpookDog]

You really need to check the back of the piston that isn’t exposed to exaust gas at the manifold. The piston skirt is exposed to the ex manifold on the upstroke.