KX100 more low end?

So, I traded my sons xr100 for a 2001 kx100. He hates it! We need to figure out how to tune the power band down lower. We would like a little bit more response on the bottom without having to rev out just to get going. Its got an fmf exhaust system. I love the bike! With a little whipping, it just screams and pulls hard! Shouldda got my son a 4 stroke,, but this is what we have,, trying to make it work.

I don't think you are not gonna get a huge increase in lowend power. You have to rev 2 strokes, that's the bottom line. But you can make the bike a little easier for him to ride and more resistant to stalling by adding a flywheel weight.

flywheel weight, oem pipe.

ofgofg. My daughter moved from a '06 CRF100 to an '09 KX100...so I went through the same issues.

You can gear it down by going with a smaller front sprocket or a larger rear sprocket or a combination of both. You might see what you have on it right now. For example, if the front sprocket is currently a 14 tooth, you can go down to a 13 tooth. It will remove some top speed, but give it more climbing power at lower rpms. Or, if it has a rear sprocket of 49, you could go up to a 52. Front sprockets are cheaper and one tooth reduction in the front sprocket is approx equal to a 3 tooth increase in the rear sprocket. Please note that if you gear it down, it will tend to lift the front wheel easier if he really gets into the throttle or doesnt have good throttle control. My guess is he is he is probably intimidated by the explosive power of the two stroke, plus this bike has about 28 hp (all concentrated in a narrower rpm range) vs. about 9 HP for the XR100.

You can also look at a flywheel weight that can give a bit more ability to lug it at the low end and will lessen the explosiveness of a two stroke. I used a Steahly 10 oz flywheel weight, but they come in 8 oz, 10 oz and 12 oz. Here is the link to their flywheel recommendation page.


It those mods dont' do the trick you might look to some cylinder porting and powervalve mods that might give it more low and mid-range. One of several firms that perform that type of work is Eric Gorr (www.ericgorr.com). I had him do some work on my sons YZ85 to give it more low end, but did not need to do that with my daughter's KX100 (the sprocket and flywheel changes seemed to do the trick).

Eric can help, he can also get you the JMS powervalve. eric is relatively cheap and the gearing thing works well too. did it for my daughter.

if you want alot more bottom end, I highly suggest a stroker. Stay stock bore with +2mm stroke on crank. get a Phathead (have domes cut) and you'll be surprised. Do a search on here and you'll find quite a few threads on here and the ones that have done it.

I can personally vouch what a stroker has. Built one for my son and he pulled on a 150RB bored/stroked to 174 with P&P head. and would easily pull on 125s and 250f's. It even went back to the stock jetting with clip up to 2nd notch.

Oh, and if you go overbore, you will need the PV cut, jetting will be very tempermental etc. I've been there/done that. from 100 to 112 and all inbetween.

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