'03 CR280 finally together.

[smarttoys]

I finally got my motor mods done on my 2003 CR250 :yeehaw: .

I went with a longer 76mm stroke (crankworks) and a 2mm over cylinder (EG) for a whopping 280cc. I haven't been able to really ride it much but it runs great in the parking lot. I can't wait til spring so I can get out on a track and fine tune the power delivery and jetting to my liking. In the mean time I better get into shape so I can hold on to it. ;)

I was in such a hurry to get the work done that forgot to dyno it before I tore it down. I'm going to try and find a stock '03 CR250 to dyno so I can get some good comparisons for myself.

 

[luvtolean]

Sweet:D I bet you won't need lower gearing or the clutch lever to get it on the pipe anymore! I've been thinking of letting Eric do his treatment on my 03 CR, and I just don't think I could hang onto it yet.

 

[G92Joe]

Did you get any porting done with it?

 

[EricGorr]

Yes porting work was performed to the cylinder. In fact we spent some time on the computer trying to juggle a bunch of dimensions that get compromised when you bore and stroke an engine. Rather than cut the head deep for the piston travel, Cometic made us an aluminum spacer plate (.063in) and I didn't need to raise the exhaust like I usually do on a port only job.
We used the 2mm oversize piston for a 2001 CR250 (702MO6850) because there is no oversize for the 2002.
It should be an interesting bike to ride, although I think you could probably throw away the first 2 gears in the tranny!

 

[smarttoys]

Eric,
Thank you for the help on this motor and answering my questions during the process. I'm REALLY going to have a hard time waiting for spring.

The (702MO6850) piston skirt needed to be modified slightly to clear the crank at BDC. Other than that everything assembled well. So far, from what little I have ridden it, vibration isn't much different than stock.

 

[Buzz Bomb]

Get ready to buy lots of rear tires...

 

[smarttoys]

I already have about 10 rear tires in my garage. I'll need them since I'm giving my 2002 YZ250 the same bore and stroke treatment this year too. :thumb:

 

[DEANSFASTWAY]

E; Did you have to weld up the exhaust port top ? and shave the P/V ? There cant be too much room left , EH? Im thinking thats why you didnt raise the exhaust any more . Ill bet it really runs . Noone is making shorter / longer rods yet for that new style crank . I thought of doing this stroke job but with a different rod but just didnt have the time to try it out yet . With the newer Cr motors its a friendlier job of raising the cyl as theres no P/V linkage from the bottom end . Ive always had a hard time extending the links on those Yamahas that jap metal just doesnt like to be welded . With the stroke job on the CR where did you end up with ign timing? Let us know what kind of power it makes it must be really strong .

 

[smarttoys]

I advanced the timing a couple degrees to compensate for the longer stroke. I'll play with that more when I get some track time. I will probably throw it on the dyno sometime too.

I'll post again when I get a decent amount of track time on it so I can get a good feel for the power delivery. It will probably be quite awhile before that happens.

On my '02 YZ250, I'm running a 2mm shorter piston (pin to top of piston) so I don't have to raise the cylinder and modify the PV linkage. It should be interesting as well.

 

[bclapham]

Originally posted by smarttoys
On my '02 YZ250, I'm running a 2mm shorter piston (pin to top of piston) so I don't have to raise the cylinder and modify the PV linkage. It should be interesting as well.

which piston is this of which we speaketh? does the skirt length need to be shortened, or do you clearnace the crank cases?
What is the motivation for a manufacturer to make it?

also, i dont want to appear rude, :) but how much did the crank mods cost? i wonder if they will start making stroker kits of the shelf at some point.

cheers, BC

 

[smarttoys]

which piston is this of which we speaketh? does the skirt length need to be shortened, or do you clearnace the crank cases?

It is a 68.50mm piston for a '98 YZ250. The skirt may need do be shortened slightly for the increased stroke. I might need to do some reshaping of the skirt also. It will be a couple weeks before I get this one together since I'm waiting for a few parts. I'd have more details then.


I don't know what Crankworks charges retail for the crank stroking. I'm guessing $300-$350 with parts but you should contact them to find out for sure.

 

[EricGorr]

John, just to let you know I'm starting out with a new core cylinder to weld the transfers on. You can go to a maximum bore size of 70mm (677MO7000 Wiseco). I'll just trade-up for the cylinder we did previously.
I may have to relocate the inner o-ring groove for adequate margin. This will give you a bore and stroke of 70x76mm

Bruce, off-the-shelf strokers? Some companies stock stroked cranks for trade-up because of the long lead time that Crank Works has (6-8 weeks). TUF Racing has big bore cylinders and stroked cranks for the KX65, 85 & 100 but only when you send in the entire engine. Prices for stroking and new rod kits start at $350
The cost to rebuild and engine and change the stroke and bore runs about $1,000

Dean, no I didn't have to weld the exhaust, I off-set the bore .5mm from the centerline. The piston pin is off-set 1mm so you're compromising that factor a bit which may make the rings wear faster on the exhaust side.

 

[smarttoys]

Well, my YZ crank is on the way back from crankworks. Eric is working on my 70mm cylinder. I'm really excited to get this one together. The suspense is killing me. ;) With the displacement calculator I used, I should have 292cc. Fun stuff!

I feel better about increasing the bore and stroke similar amounts (3.5mm larger bore, 4mm longer stroke).

 

[G92Joe]

Smarttoys,
Any new news on your cr280 & yz290? Any dyno runs or ride reports/comparisons?

 

[smarttoys]

It's been too sloppy around here to ride much yet. I had the CR out a couple times and the power feels good so far. It has good roll on power out of the corners and revs out great. I need to get some real track time on it and I still need to tweak the jetting a bit. I would have dynoed the CR already but we are in the process of replacing our dyno computer. Once I get that up and running again I will know more about actual HP numbers.

I'm still waiting for my 70mm YZ cylinder. Right now I have the YZ motor together with my 69.50mm cylinder. I've only taken it out for a couple break-in rides. The power delivery feels smoother than the CR, like it has more torque.

 

[EricGorr]

John, I'll be shipping the 70mm YZ this week. Please advise me on the head mods regarding the difference in timing heights on the pistons vs. the stroke change.

 

[G92Joe]

Would a custom expansion chamber help with getting more HP/Tq/rpm out of these bore/stroked 250's? Who would be able/willing to fabricate it, ProCircuit? Would it be worthwhile?

 

[smarttoys]

I have a photo of my YZ250 rear wheel hp run in the members gallery area. I have to do some tuning on it but so far I very happy with the way it runs.


http://www.dirtrider.net/teamdrn/showphoto.php?photo=4399&papass=&sort=1&thecat=500

My CR still needs some tweaking before I post the runs on that.