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New to me 2003 yz125

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I've always wanted a 125. Pretty stoked on my purchase. Any suggested must do upgrades? I'm going to change out all fluids, order a few new air filters, throw a new front tire on, and order a jd jet kit.

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Fmf pipe and v force 3 reeds really opened up my 02. Also if you have the cash, a 144 kit will change how anyone feels about a 125

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Fmf pipe and v force 3 reeds really opened up my 02. Also if you have the cash, a 144 kit will change how anyone feels about a 125

Thanks. Ill add pipe and new reeds soon. I should mention my regular bike is yz250. Just picked this up for a practice bike to work on momentum and clutch practice. And to feel like a hero keeping it pinned everywhere

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Thanks. Ill add pipe and new reeds soon. I should mention my regular bike is yz250. Just picked this up for a practice bike to work on momentum and clutch practice. And to feel like a hero keeping it pinned everywhere

Haha I feel you there. I have a yz250 as well and it was down for a little bit a few weeks back so I was riding my buddies 125 all the time and it definitely felt badass to have that bike wfo all the time. It definitely did a lot to boost my confidence and also really helped me work on cornering speed because I couldn't just rely on the torque to pull me out of a lazy turn like I can on the 250. But then when I got back on my 250 again I tried to ride it like the 125 and that didn't go too well haha.

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I have also been contemplating trading my 250 for a 125 to try to make myself a faster rider instead of relying on the power to make me fast but I haven't found anything nice enough to even consider.

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I have also been contemplating trading my 250 for a 125 to try to make myself a faster rider instead of relying on the power to make me fast but I haven't found anything nice enough to even consider.

So that's my plan with the 125. Being in California, most trails anywhere close to me close from May to October, so I'm stuck riding tracks or driving almost 3 hours for dusty hard pack trails. So I plan on hitting as many tracks, there's a few fun sand tracks to choose from nearby, on the weekends and focusing on the basics while doing a full teardown on the 250. Maybe even race a few othg races on the 125. Then ride the 250 come fall hare scramble season. I'm hoping with all my woods mods I won't get into too much trouble hopping back on the bigger bike.

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So that's my plan with the 125. Being in California, most trails anywhere close to me close from May to October, so I'm stuck riding tracks or driving almost 3 hours for dusty hard pack trails. So I plan on hitting as many tracks, there's a few fun sand tracks to choose from nearby, on the weekends and focusing on the basics while doing a full teardown on the 250. Maybe even race a few othg races on the 125. Then ride the 250 come fall hare scramble season. I'm hoping with all my woods mods I won't get into too much trouble hopping back on the bigger bike.

It will all come back to you pretty quick but I would definitely recommend getting some seat time on the 250 before racing it lol. I wish I knew any trails out by me. My only option to ride is track pretty much other then the 2 local hare scrambles in October and November.

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It will all come back to you pretty quick but I would definitely recommend getting some seat time on the 250 before racing it lol. I wish I knew any trails out by me. My only option to ride is track pretty much other then the 2 local hare scrambles in October and November.

That's crummy. Where you from? Closest trails are about 1.5 hours for me, but anything worth riding is a 2 hour minimum. Tracks are the same way unfortunately. Luckily I enjoy driving.

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I live in New York. Everything is privately owned so the only train riding I really ever get to enjoy is the hare scrambles.

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my 04 had a doma expansiom chamber which is like a european procircuit but if you can get your hands on one it will open your powerband allll the way up, I used a stock reed cage with the boyesen carbon fiber reeds. it had the broadest powerband for any 125 i rode. But everyone says put a big bore in it, just spend the money on the porting job

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my 04 had a doma expansiom chamber which is like a european procircuit but if you can get your hands on one it will open your powerband allll the way up, I used a stock reed cage with the boyesen carbon fiber reeds. it had the broadest powerband for any 125 i rode. But everyone says put a big bore in it, just spend the money on the porting job

Yes! If you don't get it ported you might as well not ever punch it out. I have heard of them being slower after boring without porting and they won't last for shit. I got an Athena cylinder for mine and it was amazing but I had to run race gas and that was too expensive for me. But Eric gorr does a really good job at a good price and he can make it run on pump gas.

Sent from my Z837VL using ThumperTalk mobile app

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I would suggest you leave the motor alone.  I have an 03 and I actually prefer it to my 06 YZ 125.  The 03 has the more traditional 125 powerband, but if you keep the revs up, it will run with any 125.  All the big bore kits I have ridden have a stronger mid but a flat top-end, which totally ruins the fun (for me) of ripping on a 125.  I would get the jetting sorted on your stock motor and call it a day.  The jetting is fairly sensitive to temperature changes, so you will probably need to adjust it seasonally.  Oh yea, make sure your clutch is in good shape, as you will using it a lot. 

My recommended mods would do the suspension first.  If you are riding moto, just get it serviced, and get springs for you weight.  If you are riding woods, a revalve is worth every penny.  I actually prefer my 03 forks in the woods to my SSS bike. 

A few other points:

1. A twin air filter gives you a little more room to get normal sized hands in the airbox.

2. I found that adding 1 tooth to the rear sprocket is good.  I think a lot of folks gear down to much.  The RK520H chain is a good replacement as it is cheap and durable.

3.  Avoid O ring chains, heavy duty tubes, heavy tires, and anything else that adds mass to the driveline.  You want that motor to spin up!

4. The recommended sag is 3.5 to 3.9 inches.  I prefer mine right in middle of that range.  

5. I run an 8 plug in the woods, and a 9 plug on the track.  With a 32:1 oil ratio I do a ring swap at 50 hours and replace the piston (Forged!) at 90-100 hours.  Don't run a cast piston that long!

As for racing, I sometimes use one of the 125s for harescrambles, but I usually use my 250.  I find it not to be a problem at all to switch.  I have so much fun riding the 125s that I never ride the 250 except for races.  

Edited by rpt50
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I would suggest you leave the motor alone.  I have an 03 and I actually prefer it to my 06 YZ 125.  The 03 has the more traditional 125 powerband, but if you keep the revs up, it will run with any 125.  All the big bore kits I have ridden have a stronger mid but a flat top-end, which totally ruins the fun (for me) of ripping on a 125.  I would get the jetting sorted on your stock motor and call it a day.  The jetting is fairly sensitive to temperature changes, so you will probably need to adjust it seasonally.  Oh yea, make sure your clutch is in good shape, as you will using it a lot. 
My recommended mods would do the suspension first.  If you are riding moto, just get it serviced, and get springs for you weight.  If you are riding woods, a revalve is worth every penny.  I actually prefer my 03 forks in the woods to my SSS bike. 
A few other points:
1. A twin air filter gives you a little more room to get normal sized hands in the airbox.
2. I found that adding 1 tooth to the rear sprocket is good.  I think a lot of folks gear down to much.  The RK520H chain is a good replacement as it is cheap and durable.
3.  Avoid O ring chains, heavy duty tubes, heavy tires, and anything else that adds mass to the driveline.  You want that motor to spin up!
4. The recommended sag is 3.5 to 3.9 inches.  I prefer mine right in middle of that range.  
5. I run an 8 plug in the woods, and a 9 plug on the track.  With a 32:1 oil ratio I do a ring swap at 50 hours and replace the piston (Forged!) at 90-100 hours.  Don't run a cast piston that long!
As for racing, I sometimes use one of the 125s for harescrambles, but I usually use my 250.  I find it not to be a problem at all to switch.  I have so much fun riding the 125s that I never ride the 250 except for races.  

Awesome input. Thanks. I don't plan on doing a big bore kit as it takes away from what I am trying to get from this bike. It has newer chain and sprocket. Have two twin air filters on order from Rocky mountain atv. Hoping to be able to ride it this weekend and see what she's got. Ill definitely report back

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2 hours ago, P84ONE said:

Shift stopper if not mentioned. 

Good point, but I believe that is for 05 and 06 models.  Anybody know for sure?

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