How to clean that mess? (pistons)

Hello,

Here are some pics of my KZ1000ST's pistons:

Piston 1:

Piston 2:

Piston 3:

Piston 4:


The bike is supposed to be 34.000km, but I'm sure it got far more than that.

First, do you think all that carbone deposit is normal? I think so, but I'd like to get your opinion. There's some oil on the pistons, since I put some oil to take the compressions before opening the motor.

How could I remove that mess? I'll mainly ride the KZ in cities, and I'd like the motor temperature to be as low as possible. I'll open the motor and take the pistons out of it to clean them.

Do you think I have to touch the rings, or can I just clean the upper part of the pistons?

Did you do a compression test before you took it apart?
If so and the compression was good on all four, I wouldn't take it apart any farther then that.
Just dump a little transmission fluid on top of those pistons and let it soak for a few hours. Then take a plastic scraper and scrape off the gunk. You can use scotchbrite too, but be mindful of any small pieces of the scotchbrite falling down next to the rings.
Other than that, they look pretty normal to me.

Well, I did a compression test before opening the motor. The problem is that I did it while the motor was cold. There was too much stuff removed (carbs, exhaust, tank, etc...) that I did it that way.

Here's the result. It's probably not useful...

You lose about 3/4-1 bar by doing the compression test cold.

Looks like it needs the valve lash adjusted since two cylinders didn't do much when you added oil.

The compression is not real good, but then it's not real bad either.

One Bar equals 14.7 psi for those that don't already know.

Thanks steell. I added 1 bar to compensate for the compression being doing while the motor is cold. Here are the results, in psi :

Cylinder 4
120 -> 120 (with oil)

Cylinder 3
118 -> 135 (with oil)

Cylinder 1
118 -> 134 (with oil)

Cylinder 2
126 -> 143 (with oil)

According to the ST workshop manual, compressions should be between 128 and 156 psi. The lower limit is 100 psi and there shouldn't be more a difference larger than 14 psi between the cylinders.

As you said, "The compression is not real good, but then it's not real bad either"

I definitely will have to clean all that carbon and have a closer look at the valve.

What about the rings? Is it really useful to change them if I don't get the cylinders rebored?

(sorry for the bad English. I'm a frog eater )

Personally, I'd run two pistons up to Top Dead Center and use a hard plastic scraper to take off the carbon, then do the same on the other two pistons. I wouldn't pull the block off, and I wouldn't put new rings in it either. I think the current rings have some life in them.

But if you have lot's of money to spend on it, a Wiseco big bore kit, cams, and head porting would be fun

In my opinion (...and not to necessarily dispute steell, since he does know his stuff and is a damn good bloke!)that engine is burning oil. You asked if the carbon on the top of the piston was normal and the quick answer is NO. Certainly lots of cold running will cause buildup like that to occur but the main culprit is oil entering the combustion chamber past the oil control ring. The reason the compressions are OK is the carbon buildup is helping to seal the cylinder and also mildly raising the compression. By scraping off the carbon next to the wall of the cylinder you are risking loosing some of that sealing quality and the next problem will be oil burning witnessed from your exhaust during engine revving.
Since you have stripped the engine this far, why not just take it a little further and put in new rings and either hone it yourself or get an engine shop to do it for you. Surface the head and cut new seats and you won't regret the investment.

Post edited by: WTF!, at: 2007/03/27 23:31

If you do decide to scrape the tops off and nothing else. Have a vaccuum cleaner handy to suck up the carbon up as you are scraping to keep the chips out of the gap between the piston and the cylinder wall. It works well.

34000K equals 21,126.61 miles, and I would expect the rings to last much longer than that.

Keep in mind that oil has been poured on top of the pistons in the pics, thus giving them their oil soaked appearance.

While I do believe the piston rings are serviceable and do not require replacement, I will agree that they are not perfect and new rings and hone would result in an increase in compression.

And like I have said many times, I am a professional cheapskate

Another thing to consider is that rich mixture causes carbon build up also. My compression went from 180 to 224 from running large jets im combination with weak spark. From the pics it looks like it may be oil in your case. At tdc the the space is very little, it doesn't take much carbon to build compression to ghost levels. If the ring lands are crusty that's case of can run but not hide forever. The valve guide clearance should be checked on the heads. Kz guides are soft and have a habit of being loose, with that condition and failing seals, you have an oil burner tried and true. On the bottom side, pistons and rings take a beating from dirty oil, lots of heat generated from borderline lean mixtures, and hard running in hot weather. If you can slide a bore gauge down into the cylinder and check the diameter, it would suit you well to know the answer to that question.

Do be careful not to let _any_ carbon fall between cylinder wall and cylinder. It's highly abrasive and will do exp/tensive damage. In a few weeks I'll be lifting the head off my KZ1000P for refurbishment, and while it's off I plan to clean the cylinders as well. The trick I'll be using is to put vaseline rond the top, so the gap between cylinder and wall is sealed with the cylinder at TDC. Clean top and remove carbon, get cylinder down, and remove vaseline and any carbon that sticks to it. (Not my trick btw, read it somewhere once)

I'll remove the cylinders and then the pistons, to clean them easily. Moreover, I'll probably hone the cylinders.

I plan to immobilize the pistons when lifting the cylinders, and then putting a plastic film around the pistons to keep the carbone out of the crankcase. You know, the kind of transparent plastic film which is used for microwaves ovens. I hope that will work.