82 Kawasaki AR80 cafe w engine swap

Picked up an 82 kawasaki ar80 last summer from my work. (pic below) Stock carb had no pilot or needle adjustment, so i swapped in a 21mm mikuni to replace the stock 18 and replaced the original tires. Then i drove it around for the summer. Was a fun little bike and did around 60mph with the larger carb. However, i wanted something a little quicker.

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cleaned up and with new tires and carb.

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and here's a quick run with the new carb
http://www.youtube.com/watch?v=9M9OuEtFR7k

For an engine swap, i needed something with an angled exhaust port to clear the frame, and wanted to stay with a kawasaki power plant. I figured the kx /kdx 80 series engines would be perfect. They were angled exhaust port and still small enough to fit in the frame. The stock ar80 has a claimed 9hp @9500 rpms, although mine seemed to peak around 8500. The pipe really didn't want to go much past that. However, the kdx engine has a claimed hp of 18 and revs to around 13000 in stock trim. So i picked up a 1983 kdx80 engine along with an 84 kx80 engine.

stock engine vs kdx80 size comparison


I did a full rebuild on the kdx and kx engines and cut the old engine out.

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out of bike stock vs kdx

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I wanted to kind of go for the look of the kr50 (production racer? can't really find much info on it)

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and really loved the colors of the kr250



First thing that need to go was the stock triple tree top clamp with it's weird integrated bars. So i picked up a honda mb5 triple tree top clamp and machined a ton of material off of it.


I then threw on some clip ons. It more or less fit like it was made to.

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I chopped the rear frame to kr50 length and then replaced the stupid heavy stock foot pegs with light aluminium ones and made some custom rear engine mounts and got everything lined up.

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Then i painted the gas tank and made some kawasaki decals to throw on it. New front motor mounts were welded on and shift linkage was made and welded on. It also got a stainless braided front brake line.

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I need to make and install a pipe and seat pan. I'll be running a trail tech vapor computer and will probably do battery powered lights. It's running a 30mm oko flatslide carb in place of the stock 28mm mikuni. Future plans include a larger front brake with twin pot caliper (if my calculations are correct, it should be able to break the ton) and possibly a custom machined head with center spark plug and better squish band.

g3ssexy said:

I need to make and install a pipe and seat pan. I'll be running a trail tech vapor computer and will probably do battery powered lights.

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I'd love to do an engine swap in my MB5 and was wondering about donor engines. You mention going with battery powered lights, is this because the KDX engine doesn't have a lighting coil? I'd like to find an engine that does, but finding any donor engines around here is futile. 18hp sounds nice though

Love what you're doing here, keep it up.

correct. no lighting coil. i'm sure i could dig something up from another kawasaki that i could retrofit. i only plan on running a headlight and tail light, and both are going to be led, so i figured i'd just go with a rechargeable battery i could plug in overnight. a cr80 engine would be nice in an mb5. i had a mb5 a while ago as well.

Yeah, CR's are a common swap in the MB5. Regardless, can't find any engines around here. I'll have to live vicariously through builds like yours

What size are the clip-ons for those tiny little forks?

Also, did you have to relocate the ignition after replacing the top triple tree?

Most excellent, I'm in.

holy crap. sorry guys. didn't realize anyone was still looking at this. it didn't get much interest, so i just stopped posting. needless to say, a lot has been done to the bike since then.

ended up making a polini race seat fit by heating it up and molding it to the frame.


painted the seat and front fender and made some pads for the seat. also installed a trail tech vapor gauge.


also heated up a buell blast headlight fairing and molded it to the stock headlight.


while i was pretty happy with the performance of the air cooled motor, i figured why not throw a liquid cooled engine in there. so that's what i did. required a few new mounts to be fabricated.



brakes need a serious upgrade from the increase in power and i had been looking for a disc that was close to the mount points of the stock brake. ended up with a ninja 636 floating front caliper. was extremely close to being bolt on. just needed to take 1mm off the inside diameter, and elongate the mounting holes by 1.5mm each. was going to have to relocate the caliper, so i also upgraded to a nissin twin pot at the same time.


while doing all the brake work, decided to also powdercoat the rims.


i also disassembled the rear shock and rebuilt it and powdercoated the spring white. new twin pot caliper with mount is also pictured here.


threw it all back together.


currently working on finishing up retrofitting a kx80 aluminum swing arm.

a few more notes and some answers to questions.

i wound my own lighting coil and designed a batteryless lighting system for leds which consists of a rectifier to convert ac to dc voltage and a buck boost converter which takes any input from 3-32? volts and outputs a steady 12v.


lightened the flywheel / rotor for some improved engine response while i was at it.


as far as clipons go, the forks are super tiny 27mm. can't really find clipons for that small of forks. ended up drawing my own in solidworks and 3d printed some prototypes to make sure they worked.



and as far as ignition (assume you mean switch) i got rid of it completely while simplifying the wiring.

Yes!!! Updates! I always have loved small displacement bikes (and the art of making them faster), and was really excited at viewing your build. You have a really cool bike! Keep posting and I will keep reading ;D

haha don't think i have much more to post. i only post fairly large changes. for example, sure i cleaned the tank and filled in some dents, but i don't really post stuff like that. only major things i want to do to it would be rearsets and finish up the swing arm.

Outrageous. This critter is going to fly.

: ] haha it does.

Are you able to do a video/soundtrack of it fired up?

I am psyched on this build!!! Thank you for responding with updates; the bike looks unbelievable! How fast can you get her going?

thanks guys. i have a video from last summer with the liquid cooled engine, but before some other updates i can possibly upload.

current pipe is too long, doesn't hit the power band like it should.

Most excellent. The silencer does a good job of taking the ear bleeding factor out as well.

I love it. Thanks for reposting after so long! The video you posted is sick. That thing really rips! I wish I had as much knowledge as you