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YZ250 to YZ300WR: a build thread

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I figured I'd start my own official build thread so I can stop polluting the "What did you do to your YZ today" thread. I am waiting on the main bearings and seals, which should be in two days from now. I also ordered a new chain and some 14 and 47 tooth sprockets (DID DZ2 chain and Outlaw Racing sprockets - very impressed with those, been running them awhile now - cheap and great quality).

 

Here is the spread:

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This is a 2011 YZ250 with 75 mostly trail, some desert, and a few hare scramble total hours. I sort of hate to take everything apart because it is running so great right now and I've got about 57 hours on this Wiseco top end, still getting almost 240 psi. I was just about to change out the rings as a preventative thing, but made somewhat of a snap decision to go 300 and WR gears. I generally don't find myself needing much more power, but I could really use wide ratio gears. But the allure of a big bore conversion was too much to pass up, so I dropped my new bike savings I had going on an Athena 293 kit, a set of WR gears (out of a '98 WR400F), the case splitter and crank installer tools, gaskets and bearings and decided to build up this YZ.

 

This is my first time splitting cases on a motorcycle engine, so I will work slowly and learn as I go with my trusty shop manual and of course, Youtube.

 

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Here are the intake and transfer ports on the Athena cylinder. I won't pretend to know really anything about two stroke ports, but I do think cleaning up and smoothing the runner and port castings a little will help the flow. I'm not going to send it off and I do have a little experience and am equipped to do some blending, so I will likely clean those up myself without changing the size or shape of anything.

 

One thing I think I may do first is install the Athena top end just to get the squish numbers and then send the head off to Ron Black to make sure it's set right for 93 pump gas.

 

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Finally, here is the patient (the bike, not the dog). I love this bike - looking forward to making it even better. The puppy there is my little helper - she'll hang out all day in the garage with me usually.

 

Much more to follow!

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Awesome project. And dog!

If you get a dremel tool with a bunch of different grinding stone tips, then it's real easy to clean up those intake port edges to give the air a cleaner path in.  With a 90 deg head adapter you can also remove any plating lips on the roof edges of the transfer ports. Just be gentle and careful.

Have you assembled a YZ250 PV mechanism before? There are a couple of tricks to getting it to glide easily.

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Awesome project. And dog!
If you get a dremel tool with a bunch of different grinding stone tips, then it's real easy to clean up those intake port edges to give the air a cleaner path in.  With a 90 deg head adapter you can also remove any plating lips on the roof edges of the transfer ports. Just be gentle and careful.
Have you assembled a YZ250 PV mechanism before? There are a couple of tricks to getting it to glide easily.

Nope, first time taking the PV assembly apart, so I'll take any tips you have to offer. Otherwise, I've got a Dremel tool with the 90 deg adapter and a whole array of grinding and shaping bits and have an air die grinder for the heavier work (which you have to be really careful with in places with not a lot of material).
Gone for a couple of days for work, but I'll do the squish on the new head and hopefully get it sent off on Friday.

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Be prepared to pay double on the head. Ron doesn't like doing these. He refused at first, but I talked him into it when I offered $150. He has a great mid for it though. Set my squish to .045" and the compression is right at 200 psi, which is exactly what I wanted. Going really high on the comp with the 300 kit is really going to kill the overrev. Stock compression on my Athena kit was 260 psi, which is way too much IMO. 

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Be prepared to pay double on the head. Ron doesn't like doing these. He refused at first, but I talked him into it when I offered $150. He has a great mid for it though. Set my squish to .045" and the compression is right at 200 psi, which is exactly what I wanted. Going really high on the comp with the 300 kit is really going to kill the overrev. Stock compression on my Athena kit was 260 psi, which is way too much IMO. 

It was actually one of your posts from awhile back that made me think about sending it off. I don't want that much compression - I don't want to have to run race gas and I would like the crank to last as well. Did you run it much with the stock Athena setup? If so, did you notice much of a loss in bottom end power after the squish and volume corrections? And I'd imagine the difference between the lower compression 300 and stock YZ250 is still significant?
I'll send Ron an email and see what he says.
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I have found that if I am going to clean up ports on something like that (which looks pretty decent really),  I have learned to do it before I ever ride it the way it comes.  (that way I don't realize IF I wasted my time.)   What I would look for more is anything that looks like it might come loose.   I am interested like a lot of us are on how this one turns out.  

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Jump start. Looked like you almost have everything you need. Manual-check, WR trans-check, 300 kit-check, on-line support-check, a few bits and bobs-check, audience-check! 

Get R done!   :ride:

Edited by Gary YZDOC
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1 hour ago, RockerYZWR said:


It was actually one of your posts from awhile back that made me think about sending it off. I don't want that much compression - I don't want to have to run race gas and I would like the crank to last as well. Did you run it much with the stock Athena setup? If so, did you notice much of a loss in bottom end power after the squish and volume corrections? And I'd imagine the difference between the lower compression 300 and stock YZ250 is still significant?
I'll send Ron an email and see what he says.

I did not run it with the stock head. I just threw it on to do a squish test and check compression. Squish was .070" and the comp was around 260 psi... I'm gonna start a build thread here soon as well with a ride report since I got to break it in this past weekend. I can tell you now that you will not regret your decision. I'm not sure if you're gonna spring for the 250x cdi, but I can't imagine a 2 stroke being any smoother than this. It has as much or more than my 300xc down low, and pulls much better up top with the dep, STIC, and lower compression combo. I couldn't be happier with the bike to be honest. 

I had planned to send the cylinder off for some porting, but decided to try it out first. In glad I did, because it has more than enough power the way it is. The only thing I did was chamfer a couple sharp port edges with a pencil grinder. 

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Jump start. Looked like you almost have everything you need. Manual-check, WR trans-check, 300 kit-check, on-line support-check, a few bits and bobs-check, audience-check! 
Get R done!   :ride:

Package from Yamaha with the rest should be in tomorrow...all the project is missing is me. Be back in town Thursday night, hopefully get the squish check done then or Friday and get the head sent out by Saturday. Then I'll do the bottom end job and get everything but the head put back together and reinstalled. Provided there are no surprises or big mistakes!
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To get "X" type power from a 300 kit the head volume needs to be around 28cc, depending on local.

You should really spring for the "X" CDI and PV spring while you're at this. Seriously, as deep as you are into this, it'll make it all pay off.

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Dang, I wish I would have picked up that spring with my Yamaha parts order. Still need to read up on the removing one of the washers, but maybe I'll see if my local dealership can get that spring for me at a decent price so I don't have to pay shipping. The CDI - I'll get there. I probably have room left in the current budget for either that or a DEP pipe. Hmmmm. I actually have a spare 2010 YZ CDI I need to get rid of, wonder if any X owners would want to trade for some reason.

 

Edit...accidentally quoted myself, awesome.

 

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The spring is easy. Just put the spring in and remove one washer on the "gear" side of the spring. Smoother, but still steps up onto the pipe pretty nice.

You'll LOVE the CDI. Seriously, it's the bomb with the spring and the lowered compression. Makes 300XC owners cry.

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Get ready for your bike to be a lot harder to kick. My 285 starts right up, but I have to kick it from the top and kick it, like you mean it.

 

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Another good reason to go full "X" mode.

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22 hours ago, RockerYZWR said:

Nope, first time taking the PV assembly apart, so I'll take any tips you have to offer. Otherwise, I've got a Dremel tool with the 90 deg adapter and a whole array of grinding and shaping bits and have an air die grinder for the heavier work (which you have to be really careful with in places with not a lot of material).
Gone for a couple of days for work, but I'll do the squish on the new head and hopefully get it sent off on Friday.

During PV reassembly, even with premix oil on the valves, you might find the main rotating shaft has a slight binding feel to it when you rotate it by hand and watch the three valves open/shut.  If that happens then don't assume it's ok and use the motor. Instead you must loosen off the pinch screws on the side valve "pulleys" and let them re-seat into position where the linkage pins don't bind.  Hopefully this makes sense after you fiddle with it.  Once you have it moving very freely, then use blue loctite on those little pinch screws and re-check.

Seems that you better be gentle at first cleaning up any plating defects on the edges on the transfer roofs. I'm unsure how aggressive your grinding stones are like, but I found it to be pretty easy. Your intake port cleanup should be super easy.

Really good point by chevy on the high compression. I once had a high 245 PSI on WOT kickover with an EG broad ported 295cc kit and the low RPM hit was way too aggressive, the motor wouldn't rev as easily, and I wore out a rod big end too soon. Plus the kick starter parts get hammered. Not worth it.   245 PSI in a 295cc motor is much more brutal than 245 PSI in a stock 249cc motor.

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