<![CDATA[Piston blow by]]>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)

]]>https://dt125r.co.uk/topic/2260/piston-blow-byRSS for NodeTue, 14 Jul 2026 13:30:12 GMTSat, 23 May 2020 14:52:58 GMT60<![CDATA[Reply to Piston blow by on Sun, 24 May 2020 13:21:11 GMT]]>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.

]]>https://dt125r.co.uk/post/23567https://dt125r.co.uk/post/23567Sun, 24 May 2020 13:21:11 GMT<![CDATA[Reply to Piston blow by on Sat, 23 May 2020 20:09:55 GMT]]>As @SpookDog says you are more talking four stroke really. There doesn't look anything abnormal there

]]>https://dt125r.co.uk/post/23564https://dt125r.co.uk/post/23564Sat, 23 May 2020 20:09:55 GMT<![CDATA[Reply to Piston blow by on Sat, 23 May 2020 18:29:42 GMT]]>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

]]>https://dt125r.co.uk/post/23563https://dt125r.co.uk/post/23563Sat, 23 May 2020 18:29:42 GMT<![CDATA[Reply to Piston blow by on Sat, 23 May 2020 15:32:52 GMT]]>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...

]]>https://dt125r.co.uk/post/23558https://dt125r.co.uk/post/23558Sat, 23 May 2020 15:32:52 GMT<![CDATA[Reply to Piston blow by on Sat, 23 May 2020 15:24:34 GMT]]>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...

]]>https://dt125r.co.uk/post/23557https://dt125r.co.uk/post/23557Sat, 23 May 2020 15:24:34 GMT