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'02 kx125

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Jan 10, 2004
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#1
I'm looking for more power out of a 02 kx125. It has some bolt on parts : pro circuit pipe & silencer (shorty), delta v-force, & a dyna fx ignition. The cylinder & head is stock. The bike has very low hours on it (due to unrelated injuries)and is ridden by 16yr old c-class rider in motocross. I'm considering increasing compression by milling the head .010-.020 and would appreciate any input or suggestions. I am willing to run race gas.
 
Joined
Dec 22, 2002
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#2
i have the same bike and chased the power train for to long so i sent my cyl.,head and power valve to eric gorr. i spent $365.00 for the kx-144 big bore kit and it's well worth the cash. haven't been able to ride yet but resonse on the stand seems nothing short of incredible. if your short on cash then try gearing down and run a 51 or 52 tooth rear sprocket. you'll lose some topend speed but corner speed will increase due to ease of staying in the higher rpm range without so much clutch use. if your looking for quick horsepower drop some vp racing fuel in the tank. ultimate 2 does the trick with minor jetting changes. throttle response is unreal and overall power is increased two fold. getting the bike jetted properly and constant maintenance throughout the whole bike is key. also time in the saddle helps! good luck and any questions please ask.
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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#3
Originally posted by angus
haven't been able to ride yet but resonse on the stand seems nothing short of incredible. 
angus, read that a few times back to yourself...now don't you feel silly ;)

kpdutt, where are you looking for more power? I'd agree that a big bore kit will make a big difference in the mid-range. It will also make a big difference in your check book. Milling the head does raise compression but it also decreases displacement, affecting low-end response. It also requires race fuel.

How 'bout a simple port and polish job? This should get you more power across the entire rev range.

Here is a reality check for you. If you are a true c-class rider, more power isn't going to make you any faster, in fact, it might make you slower. Speed is about breaking late and keeping the throttle open as long as possible, staying low on the jumps and picking the best lines. It's about putting in the laps and gaining experience.

Perhaps your dollars would be better spent on help form a pro. Just something to think about.
 
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Jan 10, 2004
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#4
Thanks for the info. The rider of the bike is my son. The bike seems to have a narrow powerband, it falls off the pipe if shifted to soon and chirps if he tries to stretch a gear out. The dyna fx ignition seemed to help on the chirping but we have not had much time with it on. i droped one tooth on the front sprocket and I think that was to much, probally going to go back with stock front and add 1 or 2 teeth to rear. It would be nice to have more bottom and more top end but I do realize it is a 125.
Thanks again
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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#5
It's a 125, so the throttle has to be kept open to make power. When your son finds himself at low rpm he needs to slip the clutch to get the engine back into the sweet spot. More power is not the answer to bad technique.
 

marcusgunby

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#6
The 02 KX had bad port timing-you need to get that addressed to widen the power.The exhaust port was too big and the transfers to small i think.
 

Jman271

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Oct 18, 2001
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#7
Don't know if this helps, but here are some specs for 02 KX125,,,,
Ex-29mm from tdc
Ex-50.5mm wide, full blow width
all trans @ 42.4mm fronts @ 21mm wide, rears @ 17.9mm
boost @ 43.7mm
2 deg advance on ignition
This give the exh about 30 deg of blowdown and will help upper mid to top power. Of course good gas, reeds, and pipe that matches tune will only help,,,
 

hoosierf

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#8
I have a 2000 kx125 that I've spend over a grand to find some giddy up. To be perfectly honest, it is my opinion that the pre 2003 KX 125 is not a good platform to build a really fast 125. I honestly wouldn't put more than a couple hundered bucks into it.
 
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Dec 19, 2002
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#9
I've ridden KX's all the way up until last year when I moved to 250's and bought a Honda. I miss my KX's especially my 125. The KX 99-02 is very finickie when it comes to gearing. Speaking from experience if your son is a C rider like I was on my KX's switch back to the stock front sprocket and go 1-2 teeth more on the rear. Go 1 at a time and test. Your bolt on mods are exactly what I ran and I did have some success with them....however I am a very aggressive rider. My suggestion would be to ditch the PC pipe and go with the FMF Fatty with a Shorty PowerCore 2 or the FMF SST with the Power Core 2 Shorty. Not quite as strong on top as the PC, but way easier to ride and the V-Force on high tension more then fills in on top where the PC was. The PC setup is geared to mid-up and tired me out until I learned to ride it. The V-Force in High tension setting is a good overall package. The Dyna ignition was always a little tinny on my bike until I added a gallon of TRICK 108 leaded to every 5 gallons. Thats 1 gallon race fuel and 4 gallons 92 supreme. I think it was the lead in it that helped. I was told that the Dyna igintion was best for mod. cylinders and heads not stock. Mine was stock. I would try those mods first to save some money. However if money is not a problem. then do what I should have done and send the cylinder into Eric, he'll talk with you and develop a porting package that will suit your son best. I would modify the head for 92 supreme gas. It will definately raise the overall performance and keep costs down as far as not having to buy race gas all the time and save on top-ends b/c your not going quite so high on compression. Whatever you do ditch the PC exhaust at least until he's up to keeping the thing on the pipe. He'll love either FMF setup. Now if your really wanting an easy to ride bike I would have Eric do a 134cc AMA mod class legal overbore. Mod the head for 92 supreme pump gas. Lastly again throw on either of the FMF systems. From what I'v read and my own testing and observations the FMF systems complement and enable the big bores to breath better than the PC system. With the 134 kit it will strengthen any weak spots on the powerband and will definately add som HP. At the same time your still legal in the 125 class at all AMA CMC NMA events except for the 125 Stock class. Just two more things. 1. Is his suspension set up for him. Having your suspension dialed for you does wonders for your comfort, confidence, and speed. Having the right Valving and spring rates to put the power to the ground will do far more than any porting. 2. Get him to a riding school. Or hook up with a local pro and pay him like $25 or more for lessons. I have trained with so many different people Donnie Hansen and Josh Hansen now a part of Team Yamaha of Troy, Ryan Huffman AMA Privateer of the year in 97 rode for Primal Impulse/Suzuki in 98, he also rode for Team Planet Honda in 99 and 2000 and is now riding for a Yamaha backed team in Arenacross. These are just two examples of people I have trained under. There are more, but my point is riding correctly and with confidence does worlds of good for your speed. Even if you do mod the bike take some lessons or a class. It's a great experience for the rider and the coach/dad to understand proper technique and how to recognize problems so that they can be fixed