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KX100 Tractor Engine

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Ive been on the quest for a better trail bike over the last few years. Ive been doing hare scrambles and quite a bit of woods riding on my YZ125, and while it does pretty damn good for a MX setup, I want something with a shorter seat height, wheelbase, and lighter weight. Ive ridden a few air cooled 4stroke machines, but find them heavy, undersuspended, and outdated. A KX100 chassis seems to be exactly what I'm after, and Ive been thinking about an engine swap, but in order to save a whole bunch of fabrication and pain in the ass of another project, I thought why not mod the stock engine?

 

Theres a whole host of mods that have a massive effect on the two stroke, like port-time area and head chambering that I'll have to farm out, I can take care of the rest (carb, exhaust fab, drivetrain, FWW, etc...).

 

Basically what I want to know is if tuning the engine for extreme low-mid will be a waste in general. There's enough gnarly, tight, black timber where I've chosen the XR80 or XR100 over my 125. I can sacrifice a few ponies up top to get something rideable just off idle. Some of the better trail bikes Ive ridden are a 150cc cammed and heavy sprung KLX125, and a stock XR200, but there is quite a bit to improve upon.

 

Ive read a few threads on here about converting the 100 into a trail machine, but have yet to find anything on tailoring the power curve by way of more extreme engine mods. Anyone on here have any experience in this particular department?

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Be easier to just cut the subframe on your 125 to lower the seat height and shove the forks up the triple clamps some..... You could also lace the 125 hubs with some 19/16 wheels.

Edited by Blackwoodz

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A lot of people will add a Stealthy flywheel weight.

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I wont be ruining my 125. I'll be much better off with a dedicated trail bike.

 

I want to shift the power curve, not tone it down. Bolt on externals aren't going to cut it. I'm talking about milling the base, rechambering the head, filling and shaping the transfer ports, and fabbing a pipe.

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Ride it first and then see what you wanna do. For all you know it's perfect, I really don't think you'll need to shift the power curve all that much, but hey every rider is different.

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I wont be ruining my 125. I'll be much better off with a dedicated trail bike.

 

I want to shift the power curve, not tone it down. Bolt on externals aren't going to cut it. I'm talking about milling the base, rechambering the head, filling and shaping the transfer ports, and fabbing a pipe.

Not seeing how what I posted is ruining your 125. You could always just buy a used subframe and mod it to lower the seat height. Be a ton cheaper than what you are talking about doing with another complete bike.

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Ive been on the quest for a better trail bike over the last few years. Ive been doing hare scrambles and quite a bit of woods riding on my YZ125, and while it does pretty damn good for a MX setup, I want something with a shorter seat height, wheelbase, and lighter weight. Ive ridden a few air cooled 4stroke machines, but find them heavy, undersuspended, and outdated. A KX100 chassis seems to be exactly what I'm after, and Ive been thinking about an engine swap, but in order to save a whole bunch of fabrication and pain in the ass of another project, I thought why not mod the stock engine?

 

Theres a whole host of mods that have a massive effect on the two stroke, like port-time area and head chambering that I'll have to farm out, I can take care of the rest (carb, exhaust fab, drivetrain, FWW, etc...).

 

Basically what I want to know is if tuning the engine for extreme low-mid will be a waste in general. There's enough gnarly, tight, black timber where I've chosen the XR80 or XR100 over my 125. I can sacrifice a few ponies up top to get something rideable just off idle. Some of the better trail bikes Ive ridden are a 150cc cammed and heavy sprung KLX125, and a stock XR200, but there is quite a bit to improve upon.

 

Ive read a few threads on here about converting the 100 into a trail machine, but have yet to find anything on tailoring the power curve by way of more extreme engine mods. Anyone on here have any experience in this particular department?

I think this is a great idea and in the woods you dont go to fast usually so bottom end power sounds like a great idea to me.

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I made my KX100 into a trails/trials bike by adding FWW and geared it down to 13/60. I went smaller with pilot jet and run 60/1 premix for better low end low end response. It's great for for tight technical trails but too slow for wide open riding because of the low gearing.

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Sure, I could lower the 125 to suit, but the engine mods are quite a bit less reversible, and chopping the subframe wont really get me anywhere. I already have the forks slid up almost as far as I can, but that has nothing to do with seat height. I can turn the YZ engine into a trail plunker, and even if I could easily switch the chassis setup back for moto/desert/scramble the engine wont work how I want.

 

The main deal is, for the trails I ride, there is a whole lot of power up top that I don't use, and a whole lot down below that I would use, but isn't there. I found the chassis, but an engine with similar curves that is 5hp and 25cc short isn't an improvement. IIRC the 100 is layin down ~27hp. I can sacrifice a good chunk of that to move the powerband lower. I'd suppose one could tune the thing to be much more usable off idle and keep the hp north of 18 that would be just dandy.

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I had Eric Gorr build me a  low/mid 98 yz 144 once and it was a total tractor. My Dad had a kdx at the time to compare to and my yz was more like a kdx than an actual kdx because it revved quicker. I should have never sold that thing. In my opinion is was the best single track bike I have ever rode. I wouldn't think you would want to go any smaller than a 125 2-stroke if you want lots of low end. The bigger wheels help handling, especially over logs, rocks, and other nasty trail stuff. The crf150r motorcrosser is an interesting bike that's not full size and probably the only 4-stroke I like. 

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I had Eric Gorr build me a low/mid 98 yz 144 once and it was a total tractor. My Dad had a kdx at the time to compare to and my yz was more like a kdx than an actual kdx because it revved quicker. I should have never sold that thing. In my opinion is was the best single track bike I have ever rode. I wouldn't think you would want to go any smaller than a 125 2-stroke if you want lots of low end. The bigger wheels help handling, especially over logs, rocks, and other nasty trail stuff. The crf150r motorcrosser is an interesting bike that's not full size and probably the only 4-stroke I like.

There's an idea, look for a crf150rb

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There's a thread somewhere on here, its about someone that made an XR200 into a much more powerful, better suspended and lighter machine. Maybe this is what you've been looking for?

 

Edit: Found it!

http://www.thumpertalk.com/topic/943320-183-lb-xr200/

Edited by Euro Enduro

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I have a buddy up by me. his name is firepower....lol. He was banned here and a few other named forums some time ago.

Terry bores out the horizontal crf style motors to over 200cc and are truely tractors that rev to 9000+ rpms.

firepower minis. He can also do amazing things the the XR80/100s.

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I had Eric Gorr build me a  low/mid 98 yz 144 once and it was a total tractor. My Dad had a kdx at the time to compare to and my yz was more like a kdx than an actual kdx because it revved quicker. I should have never sold that thing. In my opinion is was the best single track bike I have ever rode. I wouldn't think you would want to go any smaller than a 125 2-stroke if you want lots of low end. The bigger wheels help handling, especially over logs, rocks, and other nasty trail stuff. The crf150r motorcrosser is an interesting bike that's not full size and probably the only 4-stroke I like. 

This answer is more along the lines of what Im looking for. Thank you.

 

Im not so sure if "lots of low end power" is exactly what im after, considering I was extremely close to slapping KX80 forks and a Fox Float on an XR100. The stock engine got me around fine, but just didn't have the power for BIG hills, granted I probably weighed 60lbs less back then.

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