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Rear sprocket size recommendations for a woods oriented 2003 CR250r

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Last winter around this time I spent a bunch of time cleaning and restoring an '03 CR250. Now that I've got a season on the bike its time to replace some worn out parts.

 

So far the bikes got the following:

  • PWK Airstriker
  • Suspension valved/sprung for me
  • FWW (not sure of the size, previous owner added)
  • Stock expansion pipe + Procircuit silencer 
  • Bark busters, skid plate, wide pegs etc..

The bike had a sprocket combo of 13-48 this past season. Which probably explains why I never touched 5th gear. So over the past few days I've done some searching on the forum and I've seen recommendations for a rear sprocket size ranging from 49-52 - quite the gap. Can anyone help me narrow it down? I'm riding anything from slow technical single track to very rocky but wider trials. 

 

 

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My 03 has a pwk and a 50T rear sprocket. No FWW but it has a Gnarly exhaust. I aint fast, but it works a little better for me in the tight trails, rough areas better than my 02 did with stock gearing.  Even without a FWW, it rarely ever stalls when really lugging. Just a little throttle, and it picks right back up ( Thanks to the PWK A/S ) . I doubt this helps ya much, but I figure I'd atleast say how mine does, for me.

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Mine has a 50t on the back.  I haven't spent a lot of time woods riding it but I felt like that sprocket helped the low end out a lot on that bike.  I would recommend that.  IMO anything bigger would take away from the top end too much.

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50t has been fantastic on my 03 cr250

I don't compete but I do a lot of woods, single track and desert.

Plenty of bottom end to stay in second gear in the tight woods and enough top to keep you happy.

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Race Hare Scrambles / Trail Riding: 14/53 tight woods and 14/50 when it opens up. Found one course last fall where i might try 14/48 this season.

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2005 CR250, 10oz. FWW, 13/49 I ride & race the same type of terrain and trails as you describe.

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Thanks guys. 

 

It sounds like the 13/50 combo is the most popular. What do you guys weigh with gear on? I'm at least 230lbs, so I'm not sure if I should factor that in and maybe find something like a 51?

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Thanks guys. 

 

It sounds like the 13/50 combo is the most popular. What do you guys weigh with gear on? I'm at least 230lbs, so I'm not sure if I should factor that in and maybe find something like a 51?

 

 

I probably weight 175 with gear on.  At your weight, you should look into some bottom end porting and keep the 13/50 combo. 

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13/51 here too. Works good for hills and mud riding in the woods. My top speed is 73mph down the road with this combo gps verified so you aren't losing speed.

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Again, thanks everyone. I'm leaning towards the 51 given my weight, the bike set up and since I barely had to touch 5th gear this past season. Either one though is going to be better than the 48 for my purposes. 

 

I have practically your setup with a bigger tank and I run 13/51 and a Rekluse. It works for me...

 

I'm looking forward to the next top end to send my head off to RB Designs.  Check them out if haven't already

 

I read about RB Designs and Eric Gorr last year while researching. The head mod is reasonably priced, does it improve lower end? or is it an all around power improvement

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13/48 is tall unless the terrain is really open. I'm a big fan of a larger CS sprocket, I try to keep a 14t up front if I can, and with that said I like a 14/53 but I'm @ 185lbs w/gear so it's not as critical for me. I had a buddy about your stature and he kept going back to a 13/51 on all his Honda's so I think you'd be fairly happy with that.

 

 


 

 The head mod is reasonably priced, does it improve lower end? or is it an all around power improvement

 

Yes. It will do both, but you'll notice it more down low. Turning down the base of the cylinder will get you there too, and focus more on the low-end which helps most riders. Just be careful 'combining' the two mods so your squish clearance doesn't get too tight.

 

If you want to do a cheap and easy test to see if you'll like it go to Cometic and order one of their 0.005" copper base gaskets. Swap in the base gasket, scotch brite the cylinder, set the PV to 12mm and go ride.

Edited by 317

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good thinking about the gasket but wouldn't be a risk running such a thin copper base gasket on this engine, as it would leak through the two water chanels?

 

I suspect to seal perfectly you must machine flat the cylinder bottom and cases prior installing such a thin gasket just to be on the safe side.

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13/48 is tall unless the terrain is really open. I'm a big fan of a larger CS sprocket, I try to keep a 14t up front if I can, and with that said I like a 14/53 but I'm @ 185lbs w/gear so it's not as critical for me. I had a buddy about your stature and he kept going back to a 13/51 on all his Honda's so I think you'd be fairly happy with that.

 

 

 

 

Yes. It will do both, but you'll notice it more down low. Turning down the base of the cylinder will get you there too, and focus more on the low-end which helps most riders. Just be careful 'combining' the two mods so your squish clearance doesn't get too tight.

 

If you want to do a cheap and easy test to see if you'll like it go to Cometic and order one of their 0.005" copper base gaskets. Swap in the base gasket, scotch brite the cylinder, set the PV to 12mm and go ride.

 

I was honestly surprised to find a 48T in the rear as it appeared the guy who i bought it from was attempting to set it up for woods, at this point I'm going to go with 13/51 - and go from there. Although this is my second 02+ CR250, I'm coming off a 4-speed 06 KX 450, and I'd be lieing if I said I didn't miss the low end grunt. But thats probably the only thing I miss from the 450.

 

 

 

set the PV to 12mm and go ride.

 

PV = Power Valve? I didn't realize there was an adjustment here to be made. I'll have to review the service manual. Do you feel I should forgo the head work and see where the .005 gasket gets me? Are there any good threads around explaining the different gasket thicknesses?

 

Thanks all again.

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48t was stock those years, a change they made in 02 with the new chassis.

 

The PV cables stretch and wear out just like any other cables and can attribute to a lazy feeling engine. I have a photo at home that shows the 12mm PV adjustment and a post somewhere, I'll see if I can dig them up for you.

 

A stock gasket is generally 0.020" which is usually how far the squish is off (usually see 0.070", target is 0.050"ish), using the thin gasket is going to give you a taste of the full mod while being completely reversible just by donating some time. Most guys are on the fence about the mod, and some usually stay there because of one reason or another. I just throw this out there to help those guys cope w/their fears (you don't even have to replace the head gasket if you don't remove the head).  If you like the results then you can go full tilt with it and have no regrets.

 

The copper isn't that hard to work with, just be sure to clean the cases/base up as best you can, I mean CLEAN; make sure you dowels aren't corroded and fit w/out binding, place a dab of sealant on the case seams and properly torque the cylinder down and that is all you need to do. I've done a few like this and they usually don't come back because they're busy being ridden. No problems with sealing, air leaks, etc.

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I was honestly surprised to find a 48T in the rear as it appeared the guy who i bought it from was attempting to set it up for woods, at this point I'm going to go with 13/51 - and go from there. Although this is my second 02+ CR250, I'm coming off a 4-speed 06 KX 450, and I'd be lieing if I said I didn't miss the low end grunt. But thats probably the only thing I miss from the 450.

 

 

PV = Power Valve? I didn't realize there was an adjustment here to be made. I'll have to review the service manual. Do you feel I should forgo the head work and see where the .005 gasket gets me? Are there any good threads around explaining the different gasket thicknesses?

 

Thanks all again.

 

 

Thinner base gasket will give you no noticeable difference in low end torque.  If you really want the motor to give you more low end, you need to have the base of the cylinder shaved by .030", then have the head cut for whatever type of fuel you want to use (pump premium or race).  Eric Gorr does this mod for $99+ shipping charges each way.  Very reasonable.  If you want his full porting package, he charges $225 for that which is about what you would pay for an aftermarket pipe, the difference being you get real results from a porting job.  The 13/51 combo probably will feel better in the woods than a 13/48 but if you got the porting done, you will love it. 

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I thought it was discussed in another thread that the 02-04 engines don't need the base cut because the squish height was already about .060 (I forget the number exactly) which would allow the head to be modified to the sweet spot squish height. but the 05-07 engines had a larger squish height of .080 and they were taking off .030 of the cylinder base to get the squish height down to .050 which would allow changes to be made at the head like the 02-04 engines.

So a thinner base gasket taking off .010 brings the squish height of your head to .050

I'll edit this as needed because I'm not for sure on these numbers but I know there was a reason I didn't want to cut the cylinder base on my 03.

Edited by WeekendRDR

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I thought it was discussed in another thread that the 02-04 engines don't need the base cut because the squish height was already about .060 (I forget the number exactly) which would allow the head to be modified to the sweet spot squish height. but the 05-07 engines had a larger squish height of .080 and they were taking off .030 of the cylinder base to get the squish height down to .050 which would allow changes to be made at the head like the 02-04 engines.

So a thinner base gasket taking off .010 brings the squish height of your head to .050

I'll edit this as needed because I'm not for sure on these numbers but I know there was a reason I didn't want to cut the cylinder base on my 03.

 

Cutting the base of the cylinder isn't a fix for the squish issues that the 02-07 models seem to have.  It is a porting option which normally adds a lot of low end torque because it changes port timing.  So a person who needs low end torque in order to have a good trail bike will normally like that mod.  Someone who is looking to keep his top end power may like to just have the head cut to fix squish and make a better combustion chamber and keep the top end.  Now on the 05-07 models, there are 2 arguments.  There is the CamP and my argument that cutting the base gives the motor a monster power curve with no loss in top end.  The other argument is that it actually does take away from top end.  I have left my 03 alone mostly because my son rides it and doesn't need a monster motor but someday soon I am going to have EG do the full porting package on it with a base cut and opening up the exhaust port to give it top end power back. 

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Cutting the base of the cylinder isn't a fix for the squish issues that the 02-07 models seem to have.  It is a porting option which normally adds a lot of low end torque because it changes port timing.

 

Cutting the base lowers everything, including the head, and partially fixes the squish. It also raises compression, unless you relieve the dome, all of which contribute to low-end power. Top end power is in exhaust port duration, knock 0.030" off the base then raise the exhaust port to your target exhaust port duration, there's your fix...low end is enhanced and top end (with the PV fully opened) is stronger. From there you get into intake port duration, transfer port duration, blowdown, etc., opening a big can of worms and all of this is open to debate. With the 0.005" copper base gasket (effectively 0.015" off the base) you'll get a taste of the mod that's completely reversible and should still run on pump gas for cheap (as in just the cost of the gasket and some time if you're willing). This isn't meant to be a permanent mod, just for those wanting to test the waters without drowning in full on port job or wanting to sacrifice the familiar traits of the bike that they're used to.

 

If 0.015" off the base does absolutely nothing as stated, but 0.030" fixes everything, one has to wonder at what point does the magic start to work?

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Cutting the base lowers everything, including the head, and partially fixes the squish. It also raises compression, unless you relieve the dome, all of which contribute to low-end power. Top end power is in exhaust port duration, knock 0.030" off the base then raise the exhaust port to your target exhaust port duration, there's your fix...low end is enhanced and top end (with the PV fully opened) is stronger. From there you get into intake port duration, transfer port duration, blowdown, etc., opening a big can of worms and all of this is open to debate. With the 0.005" copper base gasket (effectively 0.015" off the base) you'll get a taste of the mod that's completely reversible and should still run on pump gas for cheap (as in just the cost of the gasket and some time if you're willing). This isn't meant to be a permanent mod, just for those wanting to test the waters without drowning in full on port job or wanting to sacrifice the familiar traits of the bike that they're used to.

 

If 0.015" off the base does absolutely nothing as stated, but 0.030" fixes everything, one has to wonder at what point does the magic start to work?

 

I have an 03 and I put the 05 base gasket on it in order to try that mod.  IMO, it did nothing for the bottom end of my 03 but caused it to be much more finicky jetting wise.  yes, you have to modify the head if you lower the cylinder by .030" on the 02-04 models because the squish wasn't nearly as large as with the newer models.  I did the .030" cylinder modification on my 06 and the difference in the low end torque was significant.  In fact it smoothed the entire RPM range.  I like the power delivery of the 03 but it has a really soft low end compared to my modified 06 so it would be nice to get that kind of low end torque out of the 03 in the future.  I wouldn't recommend the base gasket mod, I tried it because I wanted to see if it would add a small amount of low end torque and "fix" the squish issue but it seemed to do nothing but make jetting finicky.   

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