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Old 11-19-2010, 12:29 PM   #1
rotten OP
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Buying KX100 Advice

I found a clean looking 2001 KX100 for my teenager and being a 4 stroke rider I was wondering if anyone had advice on what to look out for on a 2 stroke. I know the top end has been done (wiseco) who knows how recent. Owner reports as a "wife bike" who answered the ad. I wanted to avoid the CA red sticker.

Thanks for any advice!
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Old 11-19-2010, 01:49 PM   #2
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Is your teenager done growing? Reason I ask is because the KX100 is a tweener bike, between the typical 85 and 125 class. I was looking hard at the KX100 when my kid was on XR100 because the CR125 seemed massive. But he grew into the 125 in no time. Had I chosen the KX100 it would have been in the stable for one season tops.
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Old 11-19-2010, 02:09 PM   #3
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Yes, 14 yr Girl so much to her dismay she is done growing ~ 5'4 about same height as my wife who I figure will also enjoy the bike. She already has some time on a 4 stroke hoping 2 stroke hit won't be too much. For what I have been told kx100 is milder than CR / YZ 85.
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Old 11-19-2010, 02:46 PM   #4
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Its a fantastic bike. I rode one (older) as a kid, sister raced one very successfully, wife has one now ('03), mom has a pair of 85s (same bike essentially).

As for things to check on a used one, nothing out of the ordinary really on this bike. Overall they are virtually indestructible. Only systemic issues are on a few bikes the powervalve governor ramps weren't pressed together quite far enough and the steel balls could fly out when the governor was open all the way (max rpm). These usually just ended up in the bottom of the case and came out with the next oil change, but could on occasion bounce around long enough to do damage.

A few also had issues with the pin for the main center valve breaking and the actuator rod breaking. Kawasaki has updated parts that are more durable. I've not actually seen either break and have taken apart a few high hour KXs so I'm not sure how much of an issue it is. I updated the wife's bike anyway though for good measure. (very inexpensive).

The KX is a touch easier to ride than the CR or YZ, but milder isn't really the right term - it makes just as much power, it just has a tiny bit more low-mid so the transition to when the power really comes on isn't quite as abrupt. This is thanks mostly to a fairly effective power valve and a touch more displacement. The RMs also have quite good powervalves and are an excellent machine as well.

Being MX race bikes they have fairly large carbs and big, high ports which means they are a bit sensitive to jetting and really need to be ridden quite hard to work really well. Still, with a bit leaner than standard jetting they make surprisingly good trail/putting bikes, just don't be alarmed if you get a bit of spooge out of the exhaust.

If both girls are on the slower side, "borrow" the bike from time to time and give it a good flogging to burn the accumulated oil and carbon out of the exhaust valve and exhaust. Every time I visit my mom I take her bike for a few laps around the track and it runs much better after such treatment.

At low-mid rpm these bikes are very mild, even slow, when on the pipe they move quite quickly. I'm 160lbs and can run respectable laptimes on an outdoor, sand MX track where power is king. I'm faster on the ported 250cc two stroke of course, but still pretty quick on the 85 and 100 and always manage to pass more than a few 450's which is a hoot. If you've never ridden a small bore two stroke before be sure to sit way forward and cover the clutch the first time you whack the throttle just in case the rear tire hooks up ;) For reference, these things make as much power as a stock DR350 and literally weigh half as much. Now of course torque is way, way less than the big DR, but up top they scream.

Lower gearing may likely be useful as well for trail riding as first is semi-tall, especially considering the minimal power at low rpm.

A flywheel weight is quite effective on the newer models with the flat slide carb (I don't remember which year they went from the round to flat slide) as they rev very quickly and if traction isn't great they can be a bit tricky to keep hooked up. It doesn't really tame the bike much, but does improve traction noticeably. The bikes with the round-slide carb rev a little slower and, although I haven't tried a fww on one, I doubt it'd be an improvement from my perspective.

Depending on the prior wife, it may already be setup for trail/slower riding as opposed to MX racing.

Note that not all girls are slow, my sis for example flys on one so forgive any generalizations :)

Stock suspension is sprung for an 85-100lbs'er I think. Wife and I are heavier than that by quite a bit so her's got stiffer springs. Really made a huge improvement on suspension performance. I can run full race pace on her bike in the woods and it remains composed.
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BikePilot screwed with this post 11-19-2010 at 02:53 PM
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Old 11-19-2010, 02:53 PM   #5
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Excellent summary

They are great little bikes that can be ridden by adults on the trails quite effectively.
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Old 11-19-2010, 02:55 PM   #6
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Indeed, on a really tight HS course I'm faster on my wife's 100 than my pimped out 250...
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Old 11-19-2010, 03:05 PM   #7
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My son went from XR100 to KX100 and likes the bike alot. He turns 14 tommorow and although he isnt small he hasnt hit his growth spurt yet either.

I only went to the KX because the XR just wasnt cutting it anymore and he wasnt big enough for a 125 yet,what I really wanted was a CRF150r but didnt wanna drop that much dough on a bike he would outgrow in a couple seasons.

I picked up a clean 01 for $700 so it worked out well,should be able to sell it when the time comes for close to what I paid for it.
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Old 11-19-2010, 03:51 PM   #8
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Wow! Thanks BikePilot and fellow ADVers! what a great summary of the bike way more than I ever expected. I look forward to picking one up so I can flog it myself. I am used to my xr650 so it should be interesting The riders will be ~130 while I am 100lbs more than that! I have a 3 hour drive tomorrow to check out the bike and if it looks good in the stable it goes until Christmas.
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Old 11-19-2010, 04:42 PM   #9
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At 130lbs stiffer springs would be a worthwhile purchase - actually makes for a plusher ride as it stays up in the stroke better. Not necessary by any means - I still ride my mom's stock 85 and have managed 30' doubles (just made very sure to hit the downslope!), but the stiffer springs do make a big improvement in both plushness and bottoming resistance. I'm ~160lbs, running the stiffest springs racetech offered at the time - don't recal what kg that was though. It still doesn't make for a terribly stiff feel, even for a 100lbs'er.

You'll love flogging it about, really some of the most fun I've had on two wheels has been on the little bike. For a full size person its sort of the ultimate pit bike

I've also got an XR650R right now, its quite a different experience in every way.
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Old 11-20-2010, 06:44 PM   #10
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Well after a 370 mile roundtrip I (my daughter) am the proud owner of a very clean 2001(important in ca = green sticker) kx100. Well worth the trip from Santa Cruz to almost Yosemite. No dings in pipe, swing arm, frame. Plastics show no stress inpact marks and starts like a dream! Good number of improvements pegs, bars, pipe, stand and box of parts. Thanks for everyone's advice (bike pilot excellent post)
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Old 11-20-2010, 07:50 PM   #11
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That bike is the same as a Suz RM100. Before you flog it, consider taking the head off and measuring the ring gap. If they are ridden on the pipe, they do go through rings and it is not uncommon to break a ring and hang it up on a port.

My daughter had a KX100 as her 2nd bike and she says it was her all time favorite. I weigh 200 lbs and could easily pop 3rd and 4th gear wheelies.

There are oversize tanks available for enduros or extended trail rides. Plus there are lots of them around, parts (included used) are certainly available.

I would have a piston kit on hand. They are stupid easy to do (very kid friendly weekend project to do with a young racer/rider) and it will probably need it before you are ready to turn it over (resell, I mean).

Another KX100 guy told me that he used washers in the exhaust header to restrict the exhaust and tame the top end. I never did it, but he said that he did it for his daughter's bike.

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Old 11-20-2010, 07:55 PM   #12
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Top end is fresh but I was considering pulling the top end as suggested. I have also heard o using washers but I'll pass unless there is no other option.

As far as piston kits.... factory or wiesco?
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Old 11-20-2010, 08:08 PM   #13
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The washer is to avoid 'loop out' syndrome. Most kids figure it out after the 1st time. Some need a washer.

As far as piston kits, I had Wiseco, but that was because I found a ton of KX/RM parts at a garage sale.

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Old 11-20-2010, 08:11 PM   #14
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Sounds like you got a great buy! It is the same as the RM, or was for a few years. Suzuki and kawasaki parted ways some time ago and suzuki went back to their own design I think. If she ever fancies a yellow bike you need only buy the plastics from suzuki :)

They are super easy to work on. I've found that the top ends last astoundingly well for us, even when raced hard and ridden on a sand track and in the dunes the little KX80 easily went longer than my CR500 in fact (strange but true). I suspect it will have a lot to do with jetting, air filtration and premix type and amount. On one extremely high-hour motor that was used hard (camped out in the sand dunes for a month, rode all day every day - it was also run without coolant a few times due to a melted wp seal- and it had at least 3 full seasons on it by that time) when I tore the top end down (still running well, just figured it was overdue) the piston skirt had some small cracks starting to form. The piston clearance and ring gap were way out of spec, but compression was still in spec. KX's tend to be that way. I've never seen a broken ring in one, but suppose it could happen, though probably less likely than most MX two strokes as the ports are not insanely aggressive. Most of the time a broken ring on a two stroke is caused by too-tight a ring gap and/or not letting the bike warm up enough before flogging it. This can cause the piston to expand more quickly than the cylinder and things get too crowded in there. Not a problem so much in the last couple of decades with aluminum plated cylinders as it was with steel liners. For what its worth I've always run yamalube 2R in them, usually at 32:1 for all-around riding, I'd mix it with more oil (28:1 or even 24:1) if the bike was going to be ridden hard (dunes, sand track, me on the bike etc).

There's a team green bulletin with the specs for the power valve governor, its a trivial matter to pull it out, measure, press to the correct spec as needed and re-install. When we got my wife's KX it was missing one ball and the governor was just a hair out of spec. Found the ball in the bottom of the case, put a new one in and pressed it back to spec - all has been well ever since :)

As for the pipe, I haven't found one I like better than stock yet. Mostly because the stock one tucks in a bit better than the aftermarket ones I've had (pro circuit, bills and fmf). The Bills does give it a bit more top end and overrev which I liked, but is something only an aggressive and/or larger rider would value. Stock pipes are ultra cheap on ebay.

I've found that newer/slower riders tend to find themselves at large throttle openings and low rpms. Leaning the needle way out helps it run cleanly in such situations. This worked well for my wife and mom (both fairly conservative riders).

I've also heard about, but never tried the washer trick.

I did have an old non-powervalve cylinder bored to 107cc and change and the ports were welded up and reground such that they were a bit lower and the exhaust port smaller overall. My sister won the women's championship in our local harescramble series on that cylinder. She was quite small/light and most of the courses fairly technical. The low-mid grunt (comparatively speaking) worked well and it still had more top end than stock thanks to the displacement bump (but didn't rev as high). That punched up the bottom-mid and restricted the overrev as you'd expect. From the bottom-mid though its stronger than even the newer KX100 with the powervalve, though the newer bike will pull away at high rpms. I prefer the newer motor, my wife is pretty indifferent.
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Old 11-20-2010, 08:12 PM   #15
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The wiseco pistons are a bit stronger than oem (forged rather than cast), but require a bit more care on warmup before flogging them really hard as the pistons shrink/expand more than the cast oem unit. Its only a concern if the rider is fairly fast and goes flat out on a cold motor. For casual trail riding no worries.
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Summer 2009 Ride Report http://advrider.com/forums/showthrea...1509c&t=507038
Summer 2008 RR. http://www.advrider.com/forums/showthread.php?t=367703
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