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Detuning CR125 for trails?

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Hi all,

 

My son is almost 14 and outgrowing his CR85 (size-wise- not ability-wise).  Knowing what I know now- a 2t MX bike was not the best thing for him- even though he loves the thing.  He still never gets into the power band to make it scream after a year with it.  Not the easiest thing to ride on mostly trails and hill climbs, and hit the MX track now and then.  We're starting to do hare scrambles now.  Painfully obvious now that the thing belongs on a racetrack- even with a FWW.

 

Anyway, trying a find a good next bike for him for next season.  He recently started riding my highly modded 230F with a 37" seat height, and don't really want to get him another mini bike that he'll outgrow fast.  Don't really want to get him another 230 because it took a butt-load of $$$ to get it the way it should have been made from the start. Want something light with great suspension, but tractable power for the riding we do.  So.....

 

How about a CR125.....37" seat height (about the lowest of any full size bike I think).......But..... cut down the base and mod the head's squish deal to get the low/mid range power and kill the big hit into the powerband.  How much will this kill the top end power and/or hit?  What kind of power will this have overall compared to stock?

 

FWW?  Change the gearing?  They don't seem to make a gnarly pipe for these.  Basically, I want to turn it into an awesome woods bike.  Any help on the engine or suspension would be great.     

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Hi all,

My son is almost 14 and outgrowing his CR85 (size-wise- not ability-wise). Knowing what I know now- a 2t MX bike was not the best thing for him- even though he loves the thing. He still never gets into the power band to make it scream after a year with it. Not the easiest thing to ride on mostly trails and hill climbs, and hit the MX track now and then. We're starting to do hare scrambles now. Painfully obvious now that the thing belongs on a racetrack- even with a FWW.

Anyway, trying a find a good next bike for him for next season. He recently started riding my highly modded 230F with a 37" seat height, and don't really want to get him another mini bike that he'll outgrow fast. Don't really want to get him another 230 because it took a butt-load of $$$ to get it the way it should have been made from the start. Want something light with great suspension, but tractable power for the riding we do. So.....

How about a CR125.....37" seat height (about the lowest of any full size bike I think).......But..... cut down the base and mod the head's squish deal to get the low/mid range power and kill the big hit into the powerband. How much will this kill the top end power and/or hit? What kind of power will this have overall compared to stock?

FWW? Change the gearing? They don't seem to make a gnarly pipe for these. Basically, I want to turn it into an awesome woods bike. Any help on the engine or suspension would be great.

well i have to say i had a cr125 it was a ton of fun to ride. When i was at that point my dad got me a crf250r. I liked it i didnt have any problem riding it it was just starting it if i wasnt on flat ground. U can always go with a lowering link and slide the fork tubes up and put a low seat on it. Since u said he never really gets on the pipe on the 85 a doubt he will do it off the start on the 125. U can always put a fly wheel weight and stuff like that. If u were to go that way i would say if there is any open places for him to just ride and get used to the bike that would be best for him and on your pocket not having to buy a bike in the middle.

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If you want to get a CR125 and use it for that type of riding, I wouldn't recommend doing cylinder mods unless you know what you are doing.  You could send it to Eric Gorr and ask him to do a Low/mid porting job on it.  That would give it a bunch more low end.  The ultimate would be to get a big bore kit on it with low/mid porting, but not sure you want to spend that kind of money.

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I just bought my 14 yo a yz125 and he loves it. His riding improved dramatically. He had an 85, then a kx100 for 6 months and looked to big on it. I can't believe how easy the transition was to a man sized bike. I ride a 450 but still have a blast on a 125. We ride a mix of trails and tracks. I wouldn't detune it. Just get a good used one and let it eat.

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I think a kdx 200 would be a good option for himm.

No kidding--that's what I'd have my 14 year old riding in the woods. Or else a KTM 200

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I have successfully done mods to my 03 cr 125 motor to help the lazy low end power and smoother power delivery. But it is still not a novice woods motor. And it never will be unless you send it to a pro runner.

I second the KDX. 95-06 are all the same. FYI. The foot peg to seat distance is shorter then any of my cr's. To the point when I ride it I want lower peg or a taller seat. But the bike stands taller making ground to seat hight taller then my or their cr's.

Edited by Kirkes
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The cr125 would not be a good choice.  Having ridden several of them I can say that they have no bottom end to speak of.  He would need to keep it on the pipe to get any kind of power, which based on your original post seems to be the problem with the 85.  KDX is a proven bike but I would lean more towards a ktm150xc or 200xc.  The power on those things is basically like a 4 stroke but with the light weight of a 2-stroke.  If you don't want to fork over the 6' tall stack of cash required to buy a KTM I would go with a yz125.  They have a shocking amount of bottom end for a 125.

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well i have to say i had a cr125 it was a ton of fun to ride. When i was at that point my dad got me a crf250r. I liked it i didnt have any problem riding it it was just starting it if i wasnt on flat ground. U can always go with a lowering link and slide the fork tubes up and put a low seat on it. Since u said he never really gets on the pipe on the 85 a doubt he will do it off the start on the 125. U can always put a fly wheel weight and stuff like that. If u were to go that way i would say if there is any open places for him to just ride and get used to the bike that would be best for him and on your pocket not having to buy a bike in the middle.

Lowering link is probably a good idea for now- my bike has e-start.  Didn't think how hard it might be for him to kick start a full size bike now.  He can get both toes on the ground on my bike and I figure he'll grow a few inches by spring, but...   He only dumped it once so far- he got high-sided in the parking area going slow.

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Another vote for either the kdx 220 or 200. Plenty of low-end power(especially the 220) and the suspension will probably be more than adequate for his ability. Even then, USD fork swaps are cheap and easy to accomplish. You can easily pick up clean, low-hour kdx's for $1200.

 

The ktm 200's are nice bikes, but are more bike than he needs. 

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If you want to get a CR125 and use it for that type of riding, I wouldn't recommend doing cylinder mods unless you know what you are doing.  You could send it to Eric Gorr and ask him to do a Low/mid porting job on it.  That would give it a bunch more low end.  The ultimate would be to get a big bore kit on it with low/mid porting, but not sure you want to spend that kind of money.

I have a good shop in town that knows what they're doing.  He's done engine work on my 230 before- I'd have him do the actual machining.  We talked some about low porting our CR85 in the past.  I can do the tear down and re-assembly.

 

I read old posts about how the low porting and a big bore is better.  Does the big bore compliment the porting somehow- make up for lost hp from the porting?  Is the big bore just boring the cylinder for a bigger piston? 

 

Also wondering if porting tames the power band, but kills the power a lot on a 125- would a low ported 250 be out of the question?  Thanks.

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I think a kdx 200 would be a good option for himm.

I've read kdx's are great woods bikes.  Aren't the newest ones about 10 years old?  Looked some on craigslist, etc., but I don't see finding a decent one reasonably close anytime soon.

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I just bought my 14 yo a yz125 and he loves it. His riding improved dramatically. He had an 85, then a kx100 for 6 months and looked to big on it. I can't believe how easy the transition was to a man sized bike. I ride a 450 but still have a blast on a 125. We ride a mix of trails and tracks. I wouldn't detune it. Just get a good used one and let it eat.

Are you saying the 125 was easier to ride than the 85, and that's why his riding improved dramatically?  I've read that the smaller the 2t engine, the worse the power band hits- bigger 2t's are actually a lot easier to ride?

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I have a good shop in town that knows what they're doing.  He's done engine work on my 230 before- I'd have him do the actual machining.  We talked some about low porting our CR85 in the past.  I can do the tear down and re-assembly.

 

I read old posts about how the low porting and a big bore is better.  Does the big bore compliment the porting somehow- make up for lost hp from the porting?  Is the big bore just boring the cylinder for a bigger piston? 

 

Also wondering if porting tames the power band, but kills the power a lot on a 125- would a low ported 250 be out of the question?  Thanks.

 

Eric Gorr's big bore kit includes porting.  It will not make the motor more "Tame" it is still a high performance mx machine but with the bigger piston (more displacement) and Low/mid porting, it will have a ton of low end torque.  I have a 94 Cr125 with an Eric Gorr 144cc big bore kit, I got his "mo betta" port work on it which is his broadest smoothest power delivery port and the bike pulls the stock gearing with no problems.  i have MX'd it, I have trail rode it, it does well everywhere with stock gearing.  Most 125's need 2 bigger tooth rear sprocket to pull the gearing with any torque at all.  If you get a big bore kit, have it professionally done.  If you do porting, know what you are doing with the porting to get best results.  4 stroke performance and 2 stroke performance are completely different so make sure your guy knows what he is doing. 

 

Edit:  Bigger piston draws in air differently than the smaller one.  The porting is neccessary in order to get all of the benefit of the larger displacement motor. 

Edited by frdbtr

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Are you saying the 125 was easier to ride than the 85, and that's why his riding improved dramatically?  I've read that the smaller the 2t engine, the worse the power band hits- bigger 2t's are actually a lot easier to ride?

 

The bigger wheels make a difference.  Small wheels get upset by small obsticals that a larger wheel will roll right over with no trouble. 

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I had my 85 for 3 years. It was a little small the last year, but I rode that 85 until I could make it sing. I also tried to ride my dads CR125 that last year, and I ended up breaking my foot on it because it was too much for me to handle. So if he can, I would say let him stay on it until he can ride it hard. He will be better off in the long run

But, if he has to move up this offseason, a 125 can be made into a decent woods bike, but not a slow speed woods bike. That's not what they are built to do. CRF250Xs are a good slow woods bike, modern ergos, although they are a bit heavy.

I think the CRF230 would be a good choice, except for the suspension problems.

KDX200s are also a decent choice, but a bit outdated bike. But also not a race bike if that's what you're looking for.

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I've read kdx's are great woods bikes.  Aren't the newest ones about 10 years old?  Looked some on craigslist, etc., but I don't see finding a decent one reasonably close anytime soon.

 

07 was the last year for the kdx, which is the same last year as the cr's. 

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07 was the last year for the kdx, which is the same last year as the cr's. 

 

05 for 220 and 06 for 200 KDX variants. If you buy any 125 for woods riding consider the Honda last. Many have good luck with the Husqvarna CR125 with the 165cc big bore kit. YZ125 always a good one. KX125 03-05 had a good for woods motor (for a 125). RM125 depended alot upon the model year. 

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As or the last year of the KDX. I though it was 06 on the 200 and 05 on the 220.

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