KX500 shifter kart build

[Polar-Bus]

I've been thinking about attempting this project for years, and seeing as how with my hip arthritis being to the point I can barely swing my leg over the seat of a dirt bike, rather than sell my KX500, why not steal the mint running engine and stuff it into my CRG shifter kart !? Here's a few pics of my CR125 race kart :





The first issue is the drive sprocket. I need a KX500 spline and a 428 pitch sprocket and nobody makes one. So time to get crafty !
I machined an arbor fixture and machined the OD of a standard KX500 sprocket :





Then I bored the ID of a new CR125 kart sprocket (428 pitch) to mate with the KX500 sprocket. I chose an 18T for now as I generally like a 16T with the 125 engine so I should be in the ballpark.


And problem solved ! I now have a custom KX500 428 pitch drive sprocket. Here?s a pic of the sprocket all welded.






So the next phase is the engine mount and exhaust pipe.  I  currently have some leads on local to me custom exhaust fab shops. I want a clean fitting and functional pipe, and the OEM KX500 pipe simply won't work by design. I'm also going to dig my 500 out of storage this weekend and yank the engine. This project is gonna be INSANE    when i'm done !  I need to do some caculations on gearing and does anyone know what the upper rpm range of a KX500 is ? I'm taking a guess that peak around 7500 rpm's ?

[RoostDaddy]

Have you looked into any of the Outlaw Kart exhausts?  There are a few that do them for the KX500, but not many.  May be what you're looking for as the fit should work for you and I'm sure you already have a silencer.
 I believe most bikes peak around 6500 /  7500 max. over-rev (with the exception of a few)

[Brute]

Having raced karts for some years, This would be insane. Fun, but insane. Finding a sprint track long enough  to stretch the legs out would be a problem. Maybe it would be best not too! What kind of top end are you gearing for?

[Polar-Bus]

Having raced karts for some years, This would be insane. Fun, but insane. Finding a sprint track long enough  to stretch the legs out would be a problem. Maybe it would be best not too! What kind of top end are you gearing for?

I put all the KX500's gearing ratios into my Excel gearing caculator and looks like I need to gear about 18/24  this will theoretically pull 130 mph @ 7000 rpm. Which in the real world should pull about 115 ? Seriously how fast would you want to push a KX500 kart ? The chassis was specifically engineered for a 125cc engine so all the corner weights will now be way off with the heavy 500 engine. However I plan on adding lead ballast to help bring the chassis back to reasonably balanced. I'm really curious to see how this kart will handle as well as stand your hair on end when whacking the throttle ! LOL

[Motorrad]

[Brute]

Standing up hair should be no problem! What is the weight difference between the 125 engine and the 500? I can't imagine it is all that much? Road race kart would be able to use the power to get more top speed, but it is more about the experiment I guess. I used to do stuff like that. Not quite that extreme though!

[Polar-Bus]

I'll scale both the engines i'm guessing about a 10-15 lb diff ? I also have a Mychron4  data unit which will be a fantastic tuning device for the engine. It tells me rpm's, water temp, lap times, and EGT's . So once I dial in a sweet tune, i'll post all my data. I've always wondered how "good" (or not)  my "seat of the pants" jetting combos really are. The EGT will tell me for sure.

[kwakman]

a 500 in a kart?  you sir, are insane. I salute you.

[Polar-Bus]

I's that one of your KX500 builds ? If so that crank was screaming "enough !! shut me down !!! "    

[Polar-Bus]

a 500 in a kart?  you sir, are insane. I salute you.

I know, i'm supposed to be easier on my body from now on per my doctor's request.... don't know what to expect when giving a KX500  shifter kart and aggressive beatdown.. 

[weymouth399]

Rich

You can try out my Legend car if you want? Road course at louden, or a oval?
It's what I had to do when I couldn't ride in the dirt anymore.
Well ride the way I know how/want to, but the body won't let me anymore.
(I am glad I can still ride though)

Bob W

[Brute]

Hey weymouth399! I had a Chappy! Pretty fun little bike. I quit riding it on the street when I almost got run over by a Hearse of all things. Sure was a cute little bike though.
The old body looses more each year, and lets me know I should change priorities. It sucks. Can't EVEN think about doing the stuff I used to do. The choices get narrowed down the older I get. Glad I am still able to do quite a bit. Not quite as busted up as some. My brother in law just had a neck surgery that will change what he does and can do by a huge amount. My family had me pegged as not making it out of my 20's, and I doubled that plus. Now I actually thiink about 'what could happen' and am much more careful. Sigh...

[Motorrad]

I's that one of your KX500 builds ? If so that crank was screaming "enough !! shut me down !!! "    

Yes thats me checking Max RPM after one ride...    Not stock port timing   If as if you couldnt figure that for yourself with that number..

Wasnt trying to twist its nuts off either..  Only time It would have hit that I can think of on that ride... was rolling up to a big sand hill...   I nailed it in 3rd... and all it did was Blow the back tire away so hard to the point of kind of hydroplaining it..    and I grabbed 4th quickly..

Now you know why I am perinoid about piston weight...

[kwakman]

a 500 in a kart?  you sir, are insane. I salute you.

I know, i'm supposed to be easier on my body from now on per my doctor's request.... don't know what to expect when giving a KX500  shifter kart and aggressive beatdown.. 

you should expect a physical bashing interspersed with maniacal laughter, followed by a long lay down in a darkened room.

[Polar-Bus]

Got the engine out. Not bad for an old fart. Took me about 2-1/2 hrs. Shame on me, my swingarm pivot bolt was getting rusty and was a bit stubborn getting out. I though I antisiezed that bolt yeas ago when I checked all my s/a bearings..guess not. Got the head off and going to send it to Larry Wiechman for the decompressor mod to help "bump start" the kart easier. I also plan on  using the stock std. .4mm  head gasket (normally I run the .2mm ) . This project will be all about smooth power delivery. Piston and cyl look mint, so don't need a top end. Next step is designing an engine mount.