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Piston Wash Help.

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When I posted a picture of my spent piston while doing a rebuild, people said it was washed. As in too much fuel so the cylinder can't get hot enough to burn everything.

I only have 3 hours on my new piston but I'd like to prevent this problem from happening again if it's an issue that needs to be fixed. The jetting thread might as well be in chinese because I yet to really understand it. The only spooge I'm getting is where my head pipe meets my cylinder. But it's not as bad since I replaced exhaust seals but it's still noticeable. I'd like to say there isn't bog but if I'm in 3rd at low rpm there is some delay when cracked open but not terrible.

The bike is an 01 yz250. 40:1 Yamalube

Do I need to be running leaner? If so, what do I need to switch? Can this be done without picking up a jet kit?

Edited by Zach Gilbert

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I don't see piston wash as a problem. Better a tad too rich than a tad too lean.

It all depends what riding style you choose to use :bonk: If you ride in low RPM the pilot, a bit higher the needle and from 75% on the main jet.

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Piston wash happens when fuel moves through the ports into the combustion chamber. This cleans the areas right on the edge of the piston. Some piston wash is fine, but for optimum running conditions, excessive wash is bad.

Premix ratio (gas/oil) is independent on that. You should pick a mixture you want and need to use for your style, then jet the bike. I run 24:1 with zero spooge.

I wouldn't worry about piston wash, it's a byproduct of jetting. Pick the premix ratio you want to run and then learn to jet your bike and then you can use your piston crown for reference to fine tune it.

To start, what's your jetting? Also how do you run the bike and at what level (A, B or C class?) if applicable.

Edited by GMO397

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Do you have a picture of the piston? I've searched your posts high and low and can't turn up anything.

Like GMO said, piston wash is a handy tool, but not necessarially the place to start when tuning. Jet for performance, see where everything else ends up. Reading wash is more useful (in my opinion) for things like snowmobiles, jet skis, etc, where engine RPM is more consisitant. One good thing to check is if the wash is even from side to side, which will tell you how the ports flow compared to one another.

Probably a bit too much wash for those tuning to the ragged edge, but I'm happy with it (my last top end)

DSC00559.jpg

Not enough wash (very lean piston)

Piston1.jpg

Uneven port flow

12012-02-19_16-50-17_119.jpg

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If I were to race, I would consider myself lucky to be a C rider but I'm strictly staying woods. I just wouldn't have the endurance considering I'm around 6ft 235-40lbs. But my bike wasn't the only thing sitting over the winter so hopefully I can shed a few. Down to a size 38 from a 40 so I've made some. Summer time will bring swimming laps practically everyday. :bonk: Not to tute my own horn, but I'm faster than all my friends I ride with who are more fit than myself. I'm the only one who can take a burm half decent and get a little air. I prefer to ride a gear lower and let her scream than short shift and let it choke up.

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Here is from my old piston. Unknow hours since I bought the bike used and rode it practically all summer. But I know I put at least 30-40 hours on the motor before I rebuilt it. Jetting wise, I have no idea. I can't find any good picture files that show how to take your carb apart and identify all the different components so I have yet to mess with it. I have pretty much took my entire bike apart before besides the carb.

Edited by Zach Gilbert

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You really need to check the jet numbers and compare them to stock jetting before you could really have a conversation.

Clean it up the best you could.

Remove the carb.

Unplug it.

Unscrew the the two screws on the top to remove the slide. Pull up on the spring and plastic holder and clamp it as high as you could get it with locking type aligator clip looking tool (I forgot what they are called)

Push the cable down into the slide and move it over to release the cable from the slide.

Unscrew the needle holder from the slide, I think a 6mm socket is the correct size. Take out the needle and record the needle number and which of the 5 positions the clip is in.

Now remove the four bowl screws from the bottom of the carb and remove the bowl.

Take a measurement from the bottom of the floats to the gasket surface and write that dimension down. The tab on the floats should just barely be touching the spring loaded pin in the needle valve.

Remove the pin that holds the float in place, it should just slide right out. Shake the floats and inspect them to make sure they are not leaking.

Remove the needle valve and inspect it. the ruber cone looking thing should not be grooved.

Take out the main jet with a 6mm socket and record the number.

Take out the slow jet or pilot jet with a small flat screwdriver. Record the number.

Now spry the passages with brake cleaner the green can, and see that the jets are not plugged. Do NOT poke inside them because you could damage them.

You could remove and clean the airscrew as well but first count the exact number of turns it took to to screw in and lightly seat the screw.

Then come back and let these guys guide you in the right direction. You will also need info such as elevation, average temperature and list any modifications that were made to the engine, like after market reeds, pipe, silencer and porting. The more info you have the better. Also, check the spark plug number.

Edited by Mark6299

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This is all good information but this doesn't do me much good. I have no idea what any of these things look like. The only thing the jetting thread shows is how to set the bowl height with detailed pictures. I need some pictures of how to change other things. Possibly a youtube video. I haven't found anything good yet.

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Zach, reading your first post, I see you have three issues:

1. People say your piston looks washed.

2. You have a little bit of 2t snot at the pipe/cylinder.

3. There is a slight delay when when cracking it open in 3rd at low RPM.

I wouldn't worry too much about number one, including this post. :bonk:

I wouldn't worry about number two at all.

With number three, nearly every bike is slow to respond when opening the throttle at low RPM, especially in higher gears.

So...what are we really left with? What exactly is the problem? Does the bike run well and reliably so?

Edited by corndog

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Zach, reading your first post, I see you have three issues:

1. People say your piston looks washed.

2. You have a little bit of 2t snot at the pipe/cylinder.

3. There is a slight delay when when cracking it open in 3rd at low RPM.

I wouldn't worry too much about number one, including this post. :bonk:

I wouldn't worry about number two at all.

With number three, nearly every bike is slow to respond when opening the throttle at low RPM, especially in higher gears.

So...what are we really left with? What exactly is the problem? Does the bike run well and reliably so?

Running like a raped ape now that the top end is rebuilt. Considering I've yet to foul a plug there must not be much to worry about. Just so I know that a little too much wash can't hurt. Either way, I'm going to need to pick up on jetting. I know it could come in handy.

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This is all good information but this doesn't do me much good. I have no idea what any of these things look like. The only thing the jetting thread shows is how to set the bowl height with detailed pictures. I need some pictures of how to change other things. Possibly a youtube video. I haven't found anything good yet.

I guess I'm not sure of the problem. You're not sure how to check/change your jetting?

If you can do a top end, you're way overqualified for swapping jets: tear into that carb you'll be fine. Find the numbers on the side of the main, pilot and needle and count the slots from the top of the needle to the clip. This information will help us to tell you what we already know: that you're running very rich.

But it will give you a chance to swap to smaller jets.

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Running like a raped ape now that the top end is rebuilt. Considering I've yet to foul a plug there must not be much to worry about. Just so I know that a little too much wash can't hurt. Either way, I'm going to need to pick up on jetting. I know it could come in handy.

I bet you'd be surprised by the amount of performance you can gain from proper jetting.

Case in point, I bet your bike runs MUCH better in the winter months than it does in the heat of the summer.

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I bet you'd be surprised by the amount of performance you can gain from proper jetting.

Case in point, I bet your bike runs MUCH better in the winter months than it does in the heat of the summer.

It certainly does, I'm aware that the cold will bring leaner conditions which in turn actually helps me out. Well I'll tear into the bike this Saturday/Sunday. For now I guess let this thread die down and I'll update you guys with the numbers over the weekend.

Thanks for being patient with me, and hopefully I don't cause too much of a headache.

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I though I explained it pretty well. But here are some pictures from an RM 250 I had apart the other day. It is not exactly the same but very close. There is really nothing to it. Nothing will fall or fly out if that's what you are worried about. Look at a schematic of your carb on the TT parts store for your bike and it will show you what everything looks like. It even tells you what jet numbers are factory stick.

DSC03104.JPG

DSC03105.JPG

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Hey zach i have a question for you: is that a frog or a creature from black lagoon or exactly what? He( or she) does indeed look sad.

One thing i can add about jetting-get genuine mikuni (not just mikuni-style) jets. The cheapys are sometimes off and you can chase your tail.

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Hey zach i have a question for you: is that a frog or a creature from black lagoon or exactly what? He( or she) does indeed look sad.

One thing i can add about jetting-get genuine mikuni (not just mikuni-style) jets. The cheapys are sometimes off and you can chase your tail.

These are Keihin carbs, I would stick with Keihin jets.

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These are Keihin carbs, I would stick with Keihin jets.

Oops thats what i meant.

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Piston wash is the average running of the motor. It is useful and is needed to understand what is going on inside the motor during the burn cycle. It is just as important in dirt bikes as it is in snowmobiles, jet skis, chainsaws,,,, Piston wash will be different bases on type of oil and gas (race, high octane, leaded) used. You want a wash the same size as your pinky nail at your exhaust ports. This is extra fuel that washes back across the top of the piston (cleaning) from the exhaust. Too much wash is nothing more than wasted fuel (rich condition) and in some conditions can create a fire inside the expansion chamber. Too much wash can remove oil from lubricating the rings resulting in reduced ring life. Rich condition will reduce power output of the motor and give slow feel. Yes I would be concern with your wash and should be addressed. Use the wash to fine tune you jetting after your base line is confirmed. You have no control over even wash so don’t be concern with that. You do control with the size of wash!

Also piston wash will change with the life of motor. If your experience with your motor, reading wash, you can see when your top end is past prime and rebuild is in order well before your dead in the water

Flip side, too little wash risk burn down, hot running and baking ring oil sticking a ring. If a ring sticks you will peal your cylinder plating (if you have any), score your cylinder wall and damage your head.

Edited by weantright

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Hey zach i have a question for you: is that a frog or a creature from black lagoon or exactly what? He( or she) does indeed look sad.

One thing i can add about jetting-get genuine mikuni (not just mikuni-style) jets. The cheapys are sometimes off and you can chase your tail.

No idea what it is. Just found it on google.

I though I explained it pretty well. But here are some pictures from an RM 250 I had apart the other day. It is not exactly the same but very close. There is really nothing to it. Nothing will fall or fly out if that's what you are worried about. Look at a schematic of your carb on the TT parts store for your bike and it will show you what everything looks like. It even tells you what jet numbers are factory stick.

Mark, you explained everything clearly and well. I just had no idea what these parts looked like inside the carb. These pictures help a ton, and knowing nothing will fly out is another thing I'm happy to hear. I'll take it apart Sunday, get some pictures, and let you guys know my numbers.

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Sounds good. I was once afraid to did into fixing the seals and changing springs inside front forks until I just did it knowing that things will not just fly out. I haven't attempted valving or shim stacks yet but someday I probably will. Anyway, one you open up the top and bottom of the carb, everything will begine to make scense. Just be carefull when you spray the passages with the little red straw that comes with the brake cleaner that you don't get it in the eye. It's hard to predict exactly where it will come out of until you do it at least one time. If you don't have compressed air available I usually turn the can upside down and hit the button. Soon after you have compressed gas coming out instead of the solvent.

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