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Converting the YZ250 motor into a 250X motor?

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I'm pretty much sold on the X CDI and lower compression for the conversion. The PV spring really isn't that much of a "got-a-have" for smooth woods power, for me. But, I guess for the price it's a steal if you have to have super smooth all the way through the rev range. I wouldn't bother with the shims, just the spring, and use 3 of the stock YZ shims in the 125 configuration. That combo works best.

 

The CDI is the "got-a-have" item. The stock head can be cut for lower compression and a tighter squish, which ends up working better than the stock X head. So save your money and just get a 23cc head mod.

 

Yes, you need the head mod to get the most out of the CDI. It allows it to chug down much lower and rev out much farther. The high compression won't allow for either the chug or the higher rev. The engine fights it on both sides of the spectrum.

 

Pipes and silencers are still pretty much like a standard YZ, in how they affect the converted bike. It makes the gnarly more useful, in that it beefs up the lower part of the power, but I'm not anymore impressed with the top than a stock pipe.

 

Personally, I still like my DEP enduro and shorty set up the best. The PC2 plat and shorty have a lot of "rip", but it seemed to defeat the purpose of the conversion. Might as well run a YZ with the stock pipe lower compression.

 

That's my input on it and I'm pretty much done testing. Their may be better set ups for different folks, but I'm very happy with how my bike is working right. Very versatile power that works well from bottom to top.

 

BTW- It still reminds me of a '99 YZ250. :thumbsup:

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I'm running a GYTR flywheel and stock timing.

 

I like the flywheel because it seems to help it chug down low. It's a little jerky without it, but I wouldn't want to run much more weight than that.

 

If I was going to do anything with the timing it would be advance it, but that might hurt the long pull on top. I don't seem to giving anything up to anybody in my class for sure. And I seem to have plenty of top speed, even with the YZ gears.

 

I just got lucky and hit a home run first time at bat  ( for me anyway ) and nothing else seems to be any better. I really like the way the bike works right now. It's smooth and never seems to come on to strong for me. When I want it to go hard I can dial up some go go juice, or if I'm a running a gear or two high I can flick the clutch and twist it hard and get some zap power and make a pass.

 

With the PV spring it's a little too flat for me. It's not bad, but I like my hybrid combo better.

 

I think I might get the top three gears and try them, but I'm in no hurry. I'm just about tickled shitless with my bike right now. :ride:

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I'm pretty much sold on the X CDI and lower compression for the conversion. The PV spring really isn't that much of a "got-a-have" for smooth woods power, for me. But, I guess for the price it's a steal if you have to have super smooth all the way through the rev range. I wouldn't bother with the shims, just the spring, and use 3 of the stock YZ shims in the 125 configuration. That combo works best.

 

The CDI is the "got-a-have" item. The stock head can be cut for lower compression and a tighter squish, which ends up working better than the stock X head. So save your money and just get a 23cc head mod.

 

Yes, you need the head mod to get the most out of the CDI. It allows it to chug down much lower and rev out much farther. The high compression won't allow for either the chug or the higher rev. The engine fights it on both sides of the spectrum.

 

Pipes and silencers are still pretty much like a standard YZ, in how they affect the converted bike. It makes the gnarly more useful, in that it beefs up the lower part of the power, but I'm not anymore impressed with the top than a stock pipe.

 

Personally, I still like my DEP enduro and shorty set up the best. The PC2 plat and shorty have a lot of "rip", but it seemed to defeat the purpose of the conversion. Might as well run a YZ with the stock pipe lower compression.

 

That's my input on it and I'm pretty much done testing. Their may be better set ups for different folks, but I'm very happy with how my bike is working right. Very versatile power that works well from bottom to top.

 

BTW- It still reminds me of a '99 YZ250. :thumbsup:

 

 

I have a spare head I am getting ready to ship off to RB. So what you are saying is measure squish and send it off to get corrected, but request a final volume of 23 cc? Then it's 23 cc head + 2016 CDI = YZ Paradise?

 

Thanks for all your hard work on this, and especially for taking the time to write it all up!   :applause:

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I have a spare head I am getting ready to ship off to RB. So what you are saying is measure squish and send it off to get corrected, but request a final volume of 23 cc? Then it's 23 cc head + 2016 CDI = YZ Paradise?

 

 

 

If you take this route just be sure RB understands how you are measuring the CC volume, i.e., with or without plug, and on or off bike.

 

Another option is to buy the X head new for around 105 USD. For some reason it is cheaper than the regular YZ head. Of course then you would have a giant .105 squish gap. That's what I am running now and it is awesome, but one day I plan to try a custom head set as 218 mentions above.

 

Cheap head example:

http://www.dealercostparts.com/oemparts/a/yam/55db25c587a8660384734f08/cylinder-head

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Wow! That's a nice price! Thanks for the reference.

 

But I like the benefits of squish.

 

Hey 218, how did you measure head volume to get 23 cc?

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Hey 218, how did you measure head volume to get 23 cc?

He posted this quite a few pages back I think. I already asked him same question. I think it was plug installed.

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Guess I'm just trying to justify my head cut. But, I think the squish width and angle are the first things to take into consideration with head design. This "tub" or toroid dome shape considered to work very well with lower octane fuel.  

 

Since I like you so much, send me a head and I'll cut it any way you want. No charge.

 

How do you secure the head for cutting, with a threaded insert in the plug hole? I have a little desktop mill but have no idea how to setup a head cut.

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Isn't it funny that low compression is the new rage after so much focus on higher compression over the years? :facepalm:

 

When I first got my 2011 yz250 it was about 245 PSI, then after the EG porting/deck cut and squish correction it was pushing 260 I believe. Still ran great on pump gas though. Never a hint of detonation. Now with the (completely stock) X head and cylinder I am measuring 195 PSI.

 

As 218 already mentioned, the lower compression is helpful both down low and at the top. It takes the bark out of the low end, and allows for extended overrev.

 

But that X CDI. That really helped smooth the midrange. I absolutely love it! I don't think it had any effect at top or bottom. :thumbsup:

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I measure my heads with the plug in. 23CC with the plug in.  

 

I have a mandrel that threads into the plug hole and I turn them on my lathe. I have cut heads on a mill with a rotary table, but it's much easier on the lathe.  

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I have a spare head I am getting ready to ship off to RB. So what you are saying is measure squish and send it off to get corrected, but request a final volume of 23 cc? Then it's 23 cc head + 2016 CDI = YZ Paradise?

 

Thanks for all your hard work on this, and especially for taking the time to write it all up!   :applause:

Yeah, I'm not sure what to tell him. You can tell him 23cc or about 190psi. Whatever makes him happy.

 

But, yes, that's what I think anyway. 23cc and the X CDI makes the perfect YZ for me.

 

I like a tight squish on any head. I've always thought engine run better with a tighter squish. I've cut many heads to stock volume with an " in spec" squish and noted improved performance, fuel mileage and starting. The stock X head is ridiculous, but it's a quick and dirty way to get X power.  

 

I've also found that about 200psi on most engines starts to give diminishing returns. A lot of guys will argue this, but that's just my opinion. All of this is just my opinion. I haven't tested every combo, or half a dozen different pipes on this set up, but I've tested about every YZ250 pipe at one time or another. My personal favorite is the DEP. I just like the way it makes power and I still like the way it works on this bike.

 

I know the mags are saying things like the X has to have a Gnarly because it was just made for this motor. That's fine, but I can tell you I don't always see eye to eye with the mags. Print or online. That's their opinion. Everybody's got one, but I often find myself questioning their motivation on some of their statements.

 

I have nothing to gain or loose here, I'm just sharing my personal findings and experience. I just hope it helps you guys build the bike of your dreams cheap as possible.

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I measure my heads with the plug in. 23CC with the plug in.  

 

I have a mandrel that threads into the plug hole and I turn them on my lathe. I have cut heads on a mill with a rotary table, but it's much easier on the lathe.  

 

I have a brand new lathe waiting for its first project. This will be it.

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I just read this thread from start to finish and it is getting bookmarked for sure! 

 

I am looking at picking up a new X  for myself as well as searching for an older 250 for my 15 year old son.  I would be modifying the older bike to "X-specs" using most of the suggestions you guys have made.

 

Questions:  What year 250 should I use as a starting point?  It seems that the X CDI may work on years as far back as 2002?  I know there weren't many changes along the way, but is there a particular year cut-off I should be shopping for?

 

Thanks

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Lots of people like the 2006 and up suspension better. Otherwise, just find a bike in great condition. And you can of course change forks on the older bikes, but SSS forks are getting harder to find and more expensive.

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Lots of people like the 2006 and up suspension better. Otherwise, just find a bike in great condition. And you can of course change forks on the older bikes, but SSS forks are getting harder to find and more expensive.

 

 

Yep.  Quick search tells me 2006 and up.  The search is on!

 

Thanks

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Its a engine, NOT a motor.. motor is electric.....

what about a pesky outboard motor on a boat ? I mean nobody calls THOSE outboard engines. But totally agree with ya Edited by Kawracr 173

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Isn't it funny that low compression is the new rage after so much focus on higher compression over the years? :facepalm:

 

When I first got my 2011 yz250 it was about 245 PSI, then after the EG porting/deck cut and squish correction it was pushing 260 I believe. Still ran great on pump gas though. Never a hint of detonation. Now with the (completely stock) X head and cylinder I am measuring 195 PSI.

 

As 218 already mentioned, the lower compression is helpful both down low and at the top. It takes the bark out of the low end, and allows for extended overrev.

 

But that X CDI. That really helped smooth the midrange. I absolutely love it! I don't think it had any effect at top or bottom. :thumbsup:

You have to remember the focus of this bike has been MOTOCROSS since the begining. Those compression numbers are good for modern U.S., short, supercross style tracks. Short blasts of massive mid range.

 

The X has a whole new focus, OFFROAD. The market wants two strokes that run like a four stroke, only better. We want to be able torque these things down to a crawl, at engine speeds lower than idle and not buck or stall.

 

A 250psi engine don't want to go there, so we add huge FWW and 4 teeth to the rear sprocket trying to make it do the best it can.

 

Then we want it to rev out like a four stroke on top so we don't have to shift and can get 90mph top speed. All the while being totally seamless in the power.

 

The 250psi engine don't want to do that either. It's focused on making power in a narrow window of RPM that works best with the pipe. Maximum power all the time. Tuners even go in and lower the cylinders to try and make more power under the maximum revs the high compression allow.

 

With the lower compression you don't need a huge FWW to chug down below idle and it still allows the engine to rev freely on top. The things that have made KTM offroad bikes so popular and the euro 2 stroke enduro bikes come to life.

 

Sorry for the lecture.

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