Building a KX250 Supermoto
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Thread: Building a KX250 Supermoto

  1. #1
    Join Date
    Nov 2006
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    Maui, Hawaii
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    Default Building a KX250 Supermoto

    So I've been racing/riding four strokes now for years and am building a new track bike. I picked up a KX 250 two stroke for cheap and am building it into a supermoto track bike. It is going to look amazing and is going to be kind of a fun novelty to have in the paddock.

    Has anyone done this with a 2 stroke? What problems will arise with a two stroke? The power is huge, why don't more people utilize this type of motor?

    The parts I've ordered so far:
    -FMF Fatty pipe for low end, predictable power.
    -Shorty silencer.
    -Beringer 4 piston Stage 2 Brake kit.
    -17" Wheels
    -Pro Taper complete triples/Bars
    -Plastics.
    -New Sprocket's 14:40 ratio.

    Outside of suspension changes, am I missing something?
    Can the stock motor handle supermoto style of abuse?
    04' KX 250 Motard
    06' CRF 450 Motard
    03' Aprilia RS 50
    03' Aprilia RSVR Factory
    07' KTM Adventure

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  3. #2
    Join Date
    Sep 2004
    Location
    Utica, NY
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    307

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    I converted my '97 KDX220 over to Supermoto about 6 years ago. I ended up taking the front end from a KX250, since the KDX forks are less than ideal.

    The 2-stroke is different than my KTM's, and there are some things, like backin' it in, that are harder on the 2-stroke because there's no real engine breaking like with the 4-strokes.

  4. #3
    Join Date
    Feb 2008
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    Tulsa, OK
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    Backin it in will be easier due to less engine braking. The non-linear power delivery of the larger two strokes is why more people choose to use a four stroke for a supermoto. I still want a two stroke 250 SM though, I'll be watching this thread to see how your build turns out. It sounds like its going to be awesome!
    Last edited by DMCCOY; 06-11-2010 at 05:51 AM.

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  6. #4
    Join Date
    Nov 2006
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    Maui, Hawaii
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    Default

    Quote Originally Posted by DMCCOY View Post
    Backin it in will be easier due to less engine braking. The non-linear power delivery of the larger two strokes is why more people choose to use a four stroke for a supermoto. I still want a two stroke 250 SM though, I'll be watching this thread to see how your build turns out. It sounds like its going to be awesome!
    My Man..........I'll feed you pictures of the progress on this thread.

    Correct about the "backing it in" It was either buy a new slipper clutch for the "450" (525 between you and I) for $1200.00, Or get a whole new 2 stroke for $1500.00. Hmmmm? Engine braking problem solved.

    Strangely enough I bought the bike a day after a MRI on my knee from a crazy high side one week prior. ON A FOUR STROKE! Evidently cold tires don't care about what type of power band is dished out.

    Admittedly it will take some time to get used to the wild engine. I hate crashing as much as I hate ordering fajitas in a restaurant. Everybody stares at you.
    04' KX 250 Motard
    06' CRF 450 Motard
    03' Aprilia RS 50
    03' Aprilia RSVR Factory
    07' KTM Adventure

  7. #5
    Join Date
    Feb 2006
    Location
    Marshalltown, IA
    Posts
    341

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    I've got a cr125 2 stroke set up for SM. I'm sure there is a HUGE diff in power but in tight corners I have to slip the clutch quite a bit to get off the corner or else it bogs pretty bad. So gearing may be fairly important between different tracks and what not. I will say the 125 is super light and I can throw it around like crazy. I've got a feeling I'm going to be eating clutches pretty quick. But that is to be expected even if I were doing MX stuff.

  8. #6
    Join Date
    Nov 2006
    Location
    Maui, Hawaii
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    To my surprise the Beringer brake kit was the 6 piston, not the 4 piston caliper. Fantastic. So far the list is:
    Brakes (Master, lines, caliper, rotor.)
    Pro Taper Triples top and bottom
    Pro Taper Bars
    Arc Clutch assembly
    Throttle assembly
    Plastics
    Excel/Talon Wheels
    Cycra hand guards
    Axle Sliders
    Complete FMF Exhaust
    Renthal 40 tooth rear and 14 tooth counter sprocket
    Renthal chain.

    It is so black and beautiful, it may be a real shame this Saturday night when it goes down for the first time. Everybody wants to turn a few hot laps on the "new kid" with the strange and exciting motor.

    I'll post some pics beforehand. Everyone thinks their kid is the best looking but this one is quite handsome.
    04' KX 250 Motard
    06' CRF 450 Motard
    03' Aprilia RS 50
    03' Aprilia RSVR Factory
    07' KTM Adventure

  9. #7
    Join Date
    Mar 2005
    Location
    York SC - just below Charlotte
    Posts
    8,617

    Default

    For power I went the other way for mid to top power not bottom. FMF SST Pipe & silencer with a V force intake. In SM you will need over rev as you are WFO screaming 95% of the time.

    Be ready to play with gearing to find the sweet spot. I had a stock front and 40 tooth rear on my YZ and at the Ocalla FL track the 450's would pull me slightly coming out of turns. I put a -1 front on and could stay right with them......roughly the same as putting a 44 rear on.

    Dont be afraid to try road race rain tires. 2 strokes do not destroy tires like the 4 strokes. I picked up a used set on the wera site for cheap.


    www.MotoHose.com
    Don't bring a gun to a snowball fight!

  10. #8
    Join Date
    Nov 2006
    Location
    Maui, Hawaii
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    Great advise. Tonight was it's maiden voyage. I did keep the front sprocket with the 40 rear and the fours did pull quite a bit on the exits. Granted they are cheating with their 450's bored out to like 610's but my feather weight definitely has a chance once it's geared and tuned.

    The two stroke track time was cut short due to a front end tucking low side caused by a malfunctioning throttle cable. The bike came out of it great as it was on my back while I slid off the track on my tummy.

    The funny thing is that immediately after the crash I ran and started up the 525 which always felt pretty limber and light. Ran it onto the track, and right away I felt like I had a flat tire or something was hanging off the bike. By comparison I felt like I was riding a big heavy tractor. The KX only has like a 15lb weight savings but feels so infinitely slim and sexy to ride. I love it not for the competitiveness, but rather the all out fun factor.

    Pictures are to follow.
    04' KX 250 Motard
    06' CRF 450 Motard
    03' Aprilia RS 50
    03' Aprilia RSVR Factory
    07' KTM Adventure

  11. #9
    Join Date
    Aug 2009
    Posts
    4

    Default

    Hey nofx,

    I'm running a 2001 KX500 2T SuMo and couldn't be happier to see another 2-smoker in here! I'm running 15/40 gearing which keeps the thumpers at bay but I don't think the 250 can pull that tall of a set-up. I've tried both extremes and I'm running an FMF Gnarly Desert pipe for low-end and am happy to grunt out of corners better.
    One recommendation is to consider a flywheel weight (I use a 14 oz Steahly) to make the 2-stroke power delivery more tractable. You can get on the gas a little earlier. Let me know if you need any other bits of intel.

    Keep smokin'!
    Steev-Oh

  12. #10
    Join Date
    Jan 2009
    Location
    brooksville FL
    Posts
    5,208

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    get a good pipe guard lol. i love seeing 2 strokes on the track
    09 DRZ400sm.
    Parts broken over 50,000+ miles....Including wrecks.
    2-clutch cables *1 snapped, 1 caught before*MINOR leaking behind counter sprocket. Front brake switch assembly. Fried front master and seized the caliper.

  13. #11
    Join Date
    Aug 2009
    Posts
    4

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    Agreed darkside. I use Moose Racing's universal pipe armor - it's the thickest gauge out there but you'd better eat your Wheaties before you bend it to shape. I also found a trick little tubular water pump guard that also provides some protection at the bottom. I love my KX5 and its nuts how much attention it gets at the races.

    Keep smokin'!

  14. #12
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    Oct 2009
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    Ireland
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    Default

    pics anyone? love 2 stroke sumos

  15. #13
    Join Date
    Aug 2009
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    Here's mine!
    Sorry I only have action shots. The K5 doesn't like to be sitting still long enough to take stationary shots...
    Attached Images Attached Images

  16. #14
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    Ireland
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    sweet pics steev-oh!!!!

  17. #15
    Join Date
    Apr 2008
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    722

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    I might be doing up my 2002 YZ250, Ill have to post some picks.
    http://www.myspace.com/aurorasnowband

    05 Husaberg FS650E
    02 YZ250
    Trackday Counter: 8 / Crash Counter: 2

  18. #16
    Join Date
    Jul 2007
    Location
    Chalfont, PA
    Posts
    238

    Default

    Hey:

    Let me start my reply off by saying that I'm a two stroke junkie. I've owned tz250s, tuned for an amateur two stroke GP team (RZ350s), owned two stroke dirt bikes by the bushel and to many two stroke jet skis to count, etc.

    I've also had 2 two stroke SM bikes, a full size KTM and an RM85.

    The two strokes are really "busy" to ride, it takes a lot more effort to get them around the track. Though they make tons of power, it's tough to get the throttle open exiting a corner if you have the revs right due to the way they make their power and manic nature. The four stroke is much smoother. You also have to shift much more and "fan" the clutch constantly on the two stroke. You're so busy managing the motor that you can't concentrate on the rest of your riding.

    My RM85 is totally set up, suspension, gp tires, brakes, ergos, motor, etc and it's fast. When I jump on my friends CRF150r that has a stock motor I'm instantly turning faster lap times and doing it easier. Same with his KXF450f.

    I'm not denying that you'll have a lot of fun on the KX250, two strokes are a hell of a wild ride. The four stroke is much sweeter and definitely faster lap time wise.

    I know you're probably deep in the project by now, but most of what you have will fit a mid 2000's kxf450 or KXF250. You can pick up an early model four stroke Kawasaki for $2k or less. Bail out of the KX250 if you can and start with a four stroke. You won't regret it.

    A year ago i never would have written this, but I'm quickly becoming a four stroke convert

    I'm building a four stroke for next season.

    Good luck.

    pat
    Last edited by zook; 08-19-2010 at 07:04 AM.
    Zook
    lots of bikes............
    1974 Porsche 911-ass engined Nazi slot car
    a couple of jet ski's

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