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Mellowing a cr250 for trail/fun use

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Hi, my 16 yr old son has recently bought a very nice 01 CR250, virtually everything rebuilt including the engine,it's got a VHM race head,HGS pipe and by the way it rides maybe had a bit of porting done. It's the bike he's always fancied(and sold his CRF250 to buy it), however, it's a bit too much bike for the fun riding and trail riding he does so we want to detune it or 'mellow' it a bit.

After reading quite a bit on here and in Eric Gorrs book here is a list of things we plan on doing, if anyone has first hand experience of these mods ,or know of any other mods to increase the low to mid torque and rideability I would appreciate you sharing the info.

Lower the cylinder barrel with a thin base gasket,I'm thinking about .5mm lower,how does that sound?

Fit a thicker headgasket to compensate for the thinner base gasket and to lower the compression ratio.

Make a 1/2" thick ali spacer to fit under the exhaust valve resonance chamber.

Turn up a flywheel weight around 13 oz to fit onto the generator rotor.

Fit a shorter rear can(opposite to what I know,longer pipes normally increase low end torque in the V8 engines I play with).

Advance the ignition timing a couple of degrees.

Re-jet the carb,which is a Keihin.

Fit a one step hotter plug.

We will also look into epoxying the rear transfer ports as touched on in Gorrs book,aiming them at each other instead of at the exhaust port. Anyone have an idea how much to narrow them by,and is this done just at the ports exits?

What we are trying to achieve is a bike that is as easy to ride as his CRF250 was,we are not concerned with losing top end power as long as the low to mid power is smooth.

Actually doing the above mods is not a problem,I'm quite conversant with engine tuning, what I need to know is if the work will actually improve the bike in the direction we want. Thanks.

please,no sell the CR and buy another CRF type answers!

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You're working in the right direction. My 01 has an Eric Gorr 265 kit with his smooth powerband porting, race gas head mod, 13oz FFW, 38mm PWK carb, V Force red cage, Boyesen power valve cover, and FMF Gnarly/Turbine Core II. With my 195# on board this thing has enough torque to pull 14/47 gearing through very tight woods trails. The taller gearing stretches each gear and aids in throttle control.

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My 2003 CR250r is set up for tight woods racing. Best mods for me so far...

1. G-2 Throttle cam system about $100

2. flywheel weight $100

3. 13/50 sprockets $100

4. Boyesen rad valve $200

5. Keinhin PWK carb, carefull jetting $225

Love the bike in the woods, want it softer still? add the $620 Rekluse Pro clutch

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My 2003 CR250r is set up for tight woods racing. Best mods for me so far...

1. G-2 Throttle cam system about $100

2. flywheel weight $100

3. 13/50 sprockets $100

4. Boyesen rad valve $200

5. Keihin PWK carb, carefull jetting $225

Love the bike in the woods, want it softer still? add the $620 Rekluse Pro clutch

+1

as I understand it flywheel weight over 12 oz may not fit under the cover, or may require grinding of areas of the back of the cover.

G2

13/50 or even 51

Gnarly/shorty combo- this will be the biggest pay off in low end.

Boyesen over v force in this case, or stock cage with pro series reeds.

Keinhin - keep it as is if the jetting is clean for woods riding.

I'd forget the worm can of gaskets , motor work,and ignition timing. Rabbit holes.

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You're working in the right direction. My 01 has an Eric Gorr 265 kit with his smooth powerband porting, race gas head mod, 13oz FFW, 38mm PWK carb, V Force red cage, Boyesen power valve cover, and FMF Gnarly/Turbine Core II. With my 195# on board this thing has enough torque to pull 14/47 gearing through very tight woods trails. The taller gearing stretches each gear and aids in throttle control.

I'm on the fence with porting, this is my thought:

doesn't the porting with 14/47 gearing be the same as not porting with 13/50. it seems to me the 14/47 would hold all the added power back close to the stock equivalent with lower gearing.

comments??

My 2003 CR250r is set up for tight woods racing. Best mods for me so far...

1. G-2 Throttle cam system about $100

2. flywheel weight $100

3. 13/50 sprockets $100

4. Boyesen rad valve $200

5. Keinhin PWK carb, carefull jetting $225

Love the bike in the woods, want it softer still? add the $620 Rekluse Pro clutch

this is my srtup with the exception of the FWW, i do have the Rekluse, i had to set it at the lowest RPM and three tungsten balls due to too much slipping and overheating the oil and disk wear

I hate to use one but,

I am shopping for the FWW, I took off a TMZ 12oz when i got the bike because it wobbled so bad i thought it would wipe out my crank bearings so I will get a steahlyoffroad. I won't even sell the TMZ, Ill just throw it out.

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I am shopping for the FWW, I took off a TMZ 12oz when i got the bike because it wobbled so bad i thought it would wipe out my crank bearings so I will get a steahlyoffroad. I won't even sell the TMZ, Ill just throw it out.

Can you have it balanced at a driveshaft shop?

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Can you have it balanced at a driveshaft shop?

thought about that but by the time/money for turning and balance why not just go new and have a tighter screw on and no play at all

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In addition to you tweaking his engine, keep in mind a 4st can turn you into a lazy rider. He might have to get better with his clutch work until you get the bike to where he wants it. Then he will appreciate your efforts more to.

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Literally I think just a 13 oz fly wheel weight is plenty to start...then suspension work....you can play with engine mods later but I really don't think it's needed. He is young and will adapt easily and I think love the 2 stroke power. With a 13 oz on there the difference is dramatic. It's way smoother and easier to handle...nothing like a stock cr. I don't think it's good to totally make this 2 stroke to be completely 4 stroke boring. Put a search on TT and you'll come up with tons of threads and info everywhere about cr 250's set for trail. Check out the TT garage also . I used to do that when building up my 01 cr. It's kind of maxed out for my needs so not checking as much as before. I even think of putting a lighter than 13oz weight to get a little more liveliness in the bike. Gearing of course is something you must play with . I have a variety of sprockets that I keep and change up depending on what I ride. .... So right off the bat I'd go fww and service the suspension. The 01 handles very well and just servicing the shock and fork will do wonders for your sons confidence in my opinion.

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You're working in the right direction. My 01 has an Eric Gorr 265 kit with his smooth powerband porting, race gas head mod, 13oz FFW, 38mm PWK carb, V Force red cage, Boyesen power valve cover, and FMF Gnarly/Turbine Core II. With my 195# on board this thing has enough torque to pull 14/47 gearing through very tight woods trails. The taller gearing stretches each gear and aids in throttle control.

14/47 is equal to 13/44, nothing short of a 500 can pull a 13/44 without severe clutch abuse. Even the 13/48 that came stock on some of these bikes was a stretch. Yes , it would really delay the hit that's for sure.

http://twostrokemotocross.com/2010/12/sprockets-and-gear-ratios-what-you-should-know/

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Literally I think just a 13 oz fly wheel weight is plenty to start...then suspension work....you can play with engine mods later but I really don't think it's needed. He is young and will adapt easily and I think love the 2 stroke power. With a 13 oz on there the difference is dramatic. It's way smoother and easier to handle...nothing like a stock cr. I don't think it's good to totally make this 2 stroke to be completely 4 stroke boring. Put a search on TT and you'll come up with tons of threads and info everywhere about cr 250's set for trail. Check out the TT garage also . I used to do that when building up my 01 cr. It's kind of maxed out for my needs so not checking as much as before. I even think of putting a lighter than 13oz weight to get a little more liveliness in the bike. Gearing of course is something you must play with . I have a variety of sprockets that I keep and change up depending on what I ride. .... So right off the bat I'd go fww and service the suspension. The 01 handles very well and just servicing the shock and fork will do wonders for your sons confidence in my opinion.

I agree. Don’t do any major motor mods, especially major one to where you can’t go back. In the future when your son gets use to the bike he may want more hit in the power band. Keep it simple fly wheel weight and gearing for now, and work on the suspension and get it set up properly for him.

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Thanks for all the replies guys, I should have mentioned that my son is no stranger to 2 strokes,we rebuilt a '90 CR125 that he cut his teeth on before he bought the CRF250,and he really likes the 2 strokes,which is why he went back to them from the CRF. However,this bike is much harder to ride at low speed compared to his old CR125, I've ridden it and at lowish speeds(before it gets into the powerband),it is very 'lumpy' and sort of blubbery,that's the best I can describe it. As soon as it comes on the pipe though,it cleans up and flies,which makes me wonder if it's had some porting done? Most of my sons riding is done on quite tight trails,lots of muddy hillclimbs/logs/slippery rocks etc with just the occasional open single track or fire road. All the mods I've described are reversable ,so I'm not too worried if they don't work as we can switch back,I was just thinking of saving myself some time!

Another thing, this bike will fire up first kick from stone cold without any choke,and idles lovely-until it warms up then it won't idle. I know 2 stroke moto x bikes don't idle but to me this signifies that it is setup far too rich.I have played with the airscrew,cleaned the airfilter,blown through all the carb passages/jets etc without any appreciable difference.

Gearing was another area I intended exploring,it's stock at the moment,I was thinking of dropping a tooth off the front sprocket to help him on tight trails(I did this with his CR125 and it seemed to help). I appreciate your help guys,any more tips will be welcome.

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Reading through my own post it seems like some of the problem could be down to incorrect jetting, I'll have to check that all the relevant jets are close to stock. I assumed that the shop that built and modified this engine would correctly jet it,but....

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Thanks for all the replies guys, I should have mentioned that my son is no stranger to 2 strokes,we rebuilt a '90 CR125 that he cut his teeth on before he bought the CRF250,and he really likes the 2 strokes,which is why he went back to them from the CRF. However,this bike is much harder to ride at low speed compared to his old CR125, I've ridden it and at lowish speeds(before it gets into the powerband),it is very 'lumpy' and sort of blubbery,that's the best I can describe it. As soon as it comes on the pipe though,it cleans up and flies,which makes me wonder if it's had some porting done? Most of my sons riding is done on quite tight trails,lots of muddy hillclimbs/logs/slippery rocks etc with just the occasional open single track or fire road. All the mods I've described are reversable ,so I'm not too worried if they don't work as we can switch back,I was just thinking of saving myself some time!

Another thing, this bike will fire up first kick from stone cold without any choke,and idles lovely-until it warms up then it won't idle. I know 2 stroke moto x bikes don't idle but to me this signifies that it is setup far too rich.I have played with the airscrew,cleaned the airfilter,blown through all the carb passages/jets etc without any appreciable difference.

Gearing was another area I intended exploring,it's stock at the moment,I was thinking of dropping a tooth off the front sprocket to help him on tight trails(I did this with his CR125 and it seemed to help). I appreciate your help guys,any more tips will be welcome.

that mikuni is notoriously rich. ive got some pretty decent experience with that carb jetting my friends 01's. the main issue those carbs have is the needle jet. they come stock with an s9 needle jet which is anywhere from 2-5 sizes too big depending on where you ride.

its a pretty common mod to swap an s7 to start, and if youre like many others (including us) eventually move on to the s4 nozzle. you WILL gain throttle response. although this sounds like something that is 180* from what youre trying to accomplish for trail work, a smooth throttle response with good down low grunt (for a 2 stroke) would aid your kid as he doesnt have to crank the throttle and fan the clutch so hard to ride a narrow powerband caused by the rich jetting. if you do not feel like spending the money on a nozzle right now, leaning out the needle by raising the clip will due for now. as an example, we had to be full lean on the needle at sea level with an s7 nozzle (2 sizes leaner than stock) with float level set to stock, and the pilot was lowered down to a 27.5...like i said, IME, those carbs are rich as a pig in stock form

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14/47 is equal to 13/44, nothing short of a 500 can pull a 13/44 without severe clutch abuse.

Except a 265 tuned for low end torque and me working the clutch properly!:busted:

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I'm on the fence with porting, this is my thought:

doesn't the porting with 14/47 gearing be the same as not porting with 13/50. it seems to me the 14/47 would hold all the added power back close to the stock equivalent with lower gearing.

comments??

In this case the primary objective was to increase overall gearing in order to gain top end speed without sacrificing low speed operation. This bike is used for enduros and hard core dual sport rides. Higher gearing is needed for highway use to eliminate "screaming its guts out" with standard gearing. Another benefit to the 14/47 is smoother power output as the rpm's increase and the power valves open. This makes the bike more controllable on tight slick trails. Without the larger displacement and mods made to increase low rpm torque this gearing probably wouldn't work on low speed trail sections as 1st gear would be too high of a ratio for the engine to overcome without major clutch slippage.

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Armcosurfer, if you have a lathe, try making an extra heavy flywheel weight. If the effect is too great (which I doubt), you could later turn off some of its weight.

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Thanks guys,you are a wealth of information,we'll give the jetting,flywheel weight and gearing a go first,and try some of the other mods I mentioned later on if need be,I'll let you know how we get on.

Heart,yes I have a lathe,I was going to turn up a flywheel weight equal in weight to the heaviest that is available in the aftermarket.

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