Four Stroke Conversion For Kx80

I have a 1986 KX80 and it is too much power for my son at this time. My thought is to swap in an engine that I have from a 1974-76 Honda XR75. At this point I don't have any money invested in the KX and XR parts. He is now riding a 1970 Honda trail 70, so the step up in size and suspension and overall handeling of the KX will be an improvement. Metal fabrication wont be my problem but I'm wondering about the location of the counter sproket when the engine is mounted. It looks like it will be farther forward and even possibly slightly lower than what it is now with the KX engine. The swing arm pivot bolt runs through the back of the KX engine and that wont be possible with the XR engine so it will have to move forward or down or both. Anybody got any ideas on this?

I'm just finishing up putting a XR100 in a KX80 frame.

Email me at

and i'll try and help you out.

why not just de-tune the KX instead of turning it into junk????

because sometimes its just more satifying to make something you want, I likes to tinker.

I ride mostly in the woods and some rides have gone close to 100 miles in one day. I ride an XR400 with a 4.7 gallon tank so if me and the kid go for a long ride and he runs out of gas... dad's got a mobile filling station for him. Plus I've worked on outboards for almost 20 years so the stinky 2 stroke is getting kinda old. But I can't deny the power for the weight they have. That's what I grew up on.

Sounds like a neat project, keep us informed. You could always try a flywheel weight to tame it a bit, but I see your point about the fuel situation. Maybe look on Craigslist for an xr75 I see them on there often and pretty cheap. Either way good luck

It's def not turning it into junk. :moon:

Like I said I have a XR100 on mine, and I use it in the pitbike class. It's more fun than a KX80 could ever be.

Why do you think we have CRF150R's now? Small four stokes with suspension are a blast.

It's what keeps BBR in business Hah.

I also have TTR125 with KX forks on it. :thumbsup:

The problem with the small XR bikes is they are a bit lacking in suspension and handeling. I thought about trying to boom-down the power on the KX engine, but it's still a 2 stroke (AND THER'S NOTHING WRONG WITH THAT). But I want my kid to have a bit more mellower power. I figure if I don't have to modify the frame too much to fit the XR engine, I can put the KX engine back if he needs more power later. It's a lot of work but I'd like to teach jr. that you don't have to be stuck with what someone else builds for you.

Why not?

I ride a '99 CR250 with a hopped up KDX motor in it. works great!

Another nice combination, IMO, would be an XR250 motor in an early to mid 90's RMX frame....

For any of these projects you are obligated to put the CS sprocket where the recieving frame needs it to be. That pretty much determines where the motor is going to end up in the frame. I'd mock it up and watch out for gas tank clearence, carburator clearence and intake issues and exhaust.

If I had a spare KX100 frame I think I'd try to put an XR200 engine in it!

I've always wanted to put an XR200 engine (the older 2 valve style) in a CR125.

Like I said, The bike is a blast. I have more fun riding it than anything else.

I just need to get a bigbore kit, and Ill have a Sweet, Cheap, Low maintenance bike.

I have a spare frame you could have if you think you could get a 200 in it lol. The 100 is a very tight fit.

sounds like an interesting project...I would keep all the parts for the 2 stroke though cause as soon as he gets a little older and more experiance he will want that power.

I ride mostly in the woods and some rides have gone close to 100 miles in one day. I ride an XR400 with a 4.7 gallon tank so if me and the kid go for a long ride and he runs out of gas... dad's got a mobile filling station for him. Plus I've worked on outboards for almost 20 years so the stinky 2 stroke is getting kinda old. But I can't deny the power for the weight they have. That's what I grew up on.

That makes good sense... easy to ride, reliable, and good fuel range.. I am honestly all for engine swaps and modifying things... Sorry If I came across a bit negative in my earlier post... Its just that I aways wanted a KX80 as a kid and my parents wouldn't have any part of it... unfinished business I guess..:thumbsup:

That's a big ten-four on the childhood motorcycle denile. No harm done there CR480R. The bike I had for about twelve years before my current one was a 1981 Yamaha IT465. The only other 4stroke I ever had was a trail70 a long time ago. I guess I rethought the 2 vs 4 stroke because of the tight woods riding I mostly do. And now...... NO FOULED PLUGS!

I'm also wondering about the exhaust. (afterthoughts make any project more interesting) The stock pipe from an XR75 won't work. The short head pipe that came with the engine dosn't fit the head right so I was thinking of building my own. Stainless steel handrailing for boats is just about right. And the thickness is fine. I havn't measured it yet to be sure but it's close. It comes in a couple different sizes. Is anything available that would work good, or should I just plan on a little fab work and then buy the spark arrestor that would fit the best.

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