My piston is brown on the inside

[Kjetil]

I have a 2001 Husqvarna WR 250. I have currently a 390 main jet and Boyesen reeds. I changed the piston, and when I turn it upside down, it has a dark brown color inside under the piston top and a little bit down the sides.
Has the piston been to hot, or what cause this?

I have used this piston since summer, with a 400 jet, a 410 in the winter, and I put in a 390 a few weeks ago.

 

[MikeT]

Just latching on. I want to read all the informed responses.

 

[nikki]

I'd be curious to hear responses too. My '01 YZ 125 pistons usually were a medium-dark brown color inside the piston by the piston head but the bike ran great. Was jetted well and plugs always were a nice light tan color. BTW I usually ran Super M with 93 pump gas.

 

[MikeT]

Nikki, I run the samd gas/oil mix you do. I am curious because Marcus showed a picture of the bottom of the piston on his new KX125 and it looked brand new. I am wondering if the brown is OK, or if it is a sign of something else going on that shouldn't be.

 

[Rich Rohrich]

It's very simple, the oil cokes under the crown due to excess heat in the piston crown. It can vary from a small spot which is no big deal to a large ashy looking deposit which is usually the kiss of death.

What isn't always so simple is determining the SOURCE of the excess heat.

A few possibilities include :

- air/fuel ratio (up until power starts dropping off less fuel ends to increase combustion temperatures and running temp of the piston crown, once the mixture is TOO LEAN, somewhere past the chemically correct mixture the temps start to drop again. Rule of thumb is, best power equals highest temps.)

- engine load and the duration of the load (greater loads raise piston crown temps, and the longer the engine is under load the harder it is for the piston crown to dissipate the heat)

- coolant temperatures

- ignition advance (advancing ignition timing has a profound effect on piston crown temps)

- Pre-ignition ( a common cause in two-strokes when the wrong heat range plug is run or when a thick layer of carbon is present on the crown and forms a red hot layer of glowing deposits. Pump fuel is a large contributor to crown deposits.)

- Detonation aka knock and ping (Even mild pinging will cause a large increase in crown temperatures. As the knocking gets worse it can destroy the protective cooling layer of fuel that sits on the top of the piston crown. When this happens temps go way up in a big hurry. )

If you pay attention to the spark plug ceramic and the ground electrode you can usually see the signs of excess heat well in advance of any problems.

Most two-stroke engines run best (highest power production) when they are teetering on the brink of detonation and piston crown failure. Hard core tuners treat pistons like fuseable links. When they push to far they blow the crown out and the motor stops. :eek:
If you are dilligent about CAREFULLY tuning for conditions you can run this way with a reasonable degree of safety and make great power. For most riders this is more trouble and risk than it's worth and you'd be well served to pay attention to those warning signs and back the tuning down a bit to get a wider safety margin.
It's fun to play in that ragged edge tuning arena but it gets pretty expensive while you are learning. :thumb:

 

[bigred455]

I believe if you have color on top of the crown and also bottom of the crown it is a blow by issue. I do believe if the piston crown is too hot it will show under the crown but if you are showing color on both sides ,I bet you have blow by below the rings.

 

[marcusgunby]

Dont forget my piston was only 2 weeks old:( if i had got more time on it i bet i could have got a bit more colour on the underside;)

 

[Kjetil]

Thanks for the reply.
I think Rich is into something here. Perhaps it is the Pre-ignition issue.
I removed the top earlier this winter, and then the piston was pretty black on the top. Not thick, but still a bit carbon.
It looked almost like the top right piston in this image marcusgunby uploaded, only a bit more carbon:
http://www.dirtrider.net/forums3/attachment.php?postid=495861

Now this layer seem to have began to burn/flake away. And where this carbon has dissapeared, I can see fresh aluminium, not tan or brown color.
It looks almost like it has been painted with black paint, and heated until the paint starts to flake away.

 

[Rcannon]

This happened to a new cr 80 I worked on. The bike was ridden two or three times. It did not run well, but the owner left it alone due to the newness of the bike.

The ignition stopped working. Honda was cool enough to replace this under warranty. 30 minutes later the bike lost most of its compression.

You could see the carbon had started burning away from the top of the piston. Some of the carbon chunks became trapped in the exhaust port and scraped up the piston.

It looked just like you were describbing. The "burned off paint" look.