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Project Rebuild: 06 YZ 250

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So I picked up a new (to me) 06 YZ 250 a few days ago. Previous owner said it had under 20 hours on it (sure looked like it), but I decided I would rather be safe than sorry and get underway with a top end rebuild. Now I ran into some issues that I had some questions about. I am new to dirt so sorry if these are common questions. I have provided some pictures of the build and my areas of issue. First is my plug. Now it isn't totally caked, but it definitely looks as if the bike is running rich. Now this bike has not been run hard as it was ridden by an older gentleman who wanted to use it for woods. Would this clear up with some throttle, or does it need some tuning? If it does need tuning, what would you recommend I adjust? Secondly when I got the cylinder off I noticed some "rust like"substance on the crank. It looks and feels like rust. What could this be? Is it rust? Should I replace the crank or do you think this is not that big of a deal? The crank looks good, minus these "spots". There was a small spot that had a small blueish tint which I imagine is from running too hot.

Here is how she looked when I got her home

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Ran out of light after getting the cylinder off, which looks in great shape by the way.

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Grabbed a few quick shots of the piston

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Here is a look at my plug

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Here is the crank issue I was describing

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everything looks wet like theres little watter droplets on everything. was it like that when you took it apart or did you spill some water on it while taking it apart. if it was wet already that explaines why your crank got rusty

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everything looks wet like theres little watter droplets on everything. was it like that when you took it apart or did you spill some water on it while taking it apart. if it was wet already that explaines why your crank got rusty

I made the mistake of forgetting to drain the coolant before I removed the cylinder head.

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i was thinkng that was probably what happened. i just rebuilt my 02 and was origionally just gonna do a top end but there was a tiny bit of play in the big end bearing and the crank was a bit rusty like yours so i just replaced it. im not sure how much the rust will make a differance the bike ran perfect when i took it apart. then again i dont race but i do run it wide open most of the time. also the blue spots is just from them heating the crank at the factory to press that pin in. i was wondering about them too untill i got the new one and it looked the same. as for your jetting it was probably fine for riding on the pipe but a bit ritch maybe for putting around. you should just make sure its jetted peoperly when your done rebuilding

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there was some terrible spelling and grammar in that lol. *jetted properly

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i was thinkng that was probably what happened. i just rebuilt my 02 and was origionally just gonna do a top end but there was a tiny bit of play in the big end bearing and the crank was a bit rusty like yours so i just replaced it. im not sure how much the rust will make a differance the bike ran perfect when i took it apart. then again i dont race but i do run it wide open most of the time. also the blue spots is just from them heating the crank at the factory to press that pin in. i was wondering about them too untill i got the new one and it looked the same. as for your jetting it was probably fine for riding on the pipe but a bit ritch maybe for putting around. you should just make sure its jetted peoperly when your done rebuilding

Thank you for your input it was a huge help. This bike was running absolutely perfectly before I tore it apart. Started cold second kick. The crank looks in great condition everywhere but these spots. I would really rather not replace it unless I absolutely have to

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Great looking bike!! I'd buy a few gaskets, put it back together, and ride that puppy.

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Great looking bike!! I'd buy a few gaskets, put it back together, and ride that puppy.

I was going to order a Wiseco piston tonight. Do you think that one is good to continue riding on? Should I re-ring it? What are your thoughts on the "spots"? Sorry for all the questions as I am a recent road race convert.

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Thank you for your input it was a huge help. This bike was running absolutely perfectly before I tore it apart. Started cold second kick. The crank looks in great condition everywhere but these spots. I would really rather not replace it unless I absolutely have to

yeah i mainly did mine because of the play in the bearing i doubt that bit of rust will hurt anything. it might even help by lowering crancase volume and increasing compression lol.

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yeah i mainly did mine because of the play in the bearing i doubt that bit of rust will hurt anything. it might even help by lowering crancase volume and increasing compression lol.

I can go with that :bonk: It is just very odd because the crank looks great everywhere but these spots. I was hoping it was some gunked up oil or something that I can just scrape off with a brillo pad or something.

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I was going to order a Wiseco piston tonight. Do you think that one is good to continue riding on? Should I re-ring it? What are your thoughts on the "spots"? Sorry for all the questions as I am a recent road race convert.

I guess it depends on how you ride. I bet you could go a year on those rings and piston if you just liked to putt around once in a while at your local spot. There's also something to be said for establishing a baseline with respect to maintenance. Now that it's in your possession, you will know exactly when it received a new top end. One thing is for sure, if I wanted to ride on that top end tomorrow, I wouldn't hesitate to put it back together and ride it.

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I was going to order a Wiseco piston tonight. Do you think that one is good to continue riding on? Should I re-ring it? What are your thoughts on the "spots"? Sorry for all the questions as I am a recent road race convert.

Replace the piston if you're worried at all. No one can look at a picture of it on the internet and tell you to run it or not.

The rust on the crank lobes is from the premix. Some oils have additives that won't rust, some don't. If the bearings are in spec, throw a new piston and rings in it and ride.

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Thanks for the input everyone. I was also looking at possibly purchasing a flywheel weight. What weight would you recommend for someone that will mainly be doing MX, but with some hare scrambles from time to time?

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I guess it depends on how you ride. I bet you could go a year on those rings and piston if you just liked to putt around once in a while at your local spot. There's also something to be said for establishing a baseline with respect to maintenance. Now that it's in your possession, you will know exactly when it received a new top end. One thing is for sure, if I wanted to ride on that top end tomorrow, I wouldn't hesitate to put it back together and ride it.

I am new to MX so it won't be ridden too hard initially, but it will be mostly MX.

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dude u got the whole thing taken apart u might as well change the top end. And yes, get the Rings as well they r like more important than the piston for compression purposes. So yeah, change the whole top end, its cheap on a 2 stroke and u got it all taken apart so u might as well just do it. get gaskets too just to make everthing super fresh itll be like having an almost new bike. as far as the crank, if u pull up and down on the crank and u dont feel any play in the bearing then dont worry about it.

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dude u got the whole thing taken apart u might as well change the top end. And yes, get the Rings as well they r like more important than the piston for compression purposes. So yeah, change the whole top end, its cheap on a 2 stroke and u got it all taken apart so u might as well just do it. get gaskets too just to make everthing super fresh itll be like having an almost new bike. as far as the crank, if u pull up and down on the crank and u dont feel any play in the bearing then dont worry about it.

I was meaning should I replace only the rings and not the piston. Obviously if I buy a new piston I will be getting rings (I think most pistons come with rings if I'm no mistaken). I will check the crank tomorrow, thank you for your input.

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haha u can get away with just rings sometimes ive done that before, but u dont truely know anything about the bikes past for certain so i would just get a fresh piston then the next time u feel the topend is due just change the rings. i would say only change just rings once tho. What i mean is that i wouldnt go thru more than 2 sets of rings per every 1 piston... but thats me.

Edited by KTMrideR146

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Well as you can see there is absolutely no scoring on the sides of the piston or the cylinder for that matter. I am not sure how the top of the piston should look, but if I can get away with rings I would rather spend the money on a skid plate and flywheel weight.

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Replace the piston if you're worried at all. No one can look at a picture of it on the internet and tell you to run it or not.

Well as you can see there is absolutely no scoring on the sides of the piston or the cylinder for that matter. I am not sure how the top of the piston should look, but if I can get away with rings I would rather spend the money on a skid plate and flywheel weight.

No one is going to be able to give you a good answer on whether you should run it or not. You are risking that the piston isn't slightly loose in the bore, shakes around and drops a skirt. A new piston is much more beneficial than a flywheel weight. Learn to ride the bike without one, I think they're useless on a mx track.

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