cr 80 shifter kart and gearing with surprise

[walker187]

I have an opportunity to buy a cr 80 dirt bike for 200 to mount on my race kart. I want to know what speeds I can reach and a good gearing ratio.

My reasons to buy. I live in the absolute middle of nowhere (northern maine) there are tons of dirt bikes for sale that I could buy but they are all top dollar. Now, I was at a local autocross event at an abandoned military base. There were some race karts there and I needed tires for my kart. I asked one of the guys if he happened to have any tires for sale. He brought out some like-new bridge stone tires and I asked how much. He says free, but I have to promise that I come next year with my kart. I pooped myself. The guy just gave away $250 tires.

I'm looking for some speed, and good handling. What kind of gearing is best for such an engine? Is it worth it to install race mods?

 

[r97]

I can't help you with gearing right now, but I bet if you looked through Google a bit you could find out some of the ratios people are running. The CR125 is a pretty standard shifter kart engine, but a CR80 would still get you going plenty fast. Here is a video to give you an idea of what you're looking at,

http://www.youtube.com/watch?v=CczJCdnSCgM

I would keep the engine stock at first and upgrade as you feel it is necessary. Hope that helps, I look forward to seeing your project put together!

 

[OzFab]

Working backwards: define "race mods"?

Great score, send me some I heard about something similar at a local kart track; a dumpster full of tyres. Unfortunately for me,I found out too late

Your gearing would be similar to that of the bike which means you should be able to reach a similar speed. Someone should chime in with more info soon.

Being a race kart, you will probably be ok but, make sure your steering is absolutely spot on.

Go for it & create a build log so we can follow what you're doing

 

[jdmkid]

im on the same boat. just bought a cr80 for 300 bucks. from what i hear they do over 100km easy. I will find out tomorrow after riding

 

[walker187]

Working backwards: define "race mods"?

Race mods as in performance race piston or rod. Jetting and stuff like that.

 

[Swabbster]

if you are on a mountain road, there are lots off off trails most places, why a racer shifter, why not a off roader,

 

[walker187]

if you are on a mountain road, there are lots off off trails most places, why a racer shifter, why not a off roader,

I already have a kx 125 that I ride. I want this so I can race it at autocross events and stuff over at an old military base.

 

[walker187]

Does anyone know what the linkage is called that connects to the j arm (shifter) on the engine? Need to make or buy one.

 

[r97]

Usually people will use a rod with threaded ends, and hiem joints (or tie-rod ends) on either end of the rod. On dune buggies people also use 1/4" push/pull cables which are flexible and easy to route, but are bit expensive. Do you have your engine mounted to a frame yet? Things like shifter linkages are smaller details that should be figured out once you have a running engine mounted to the frame. That said, when mounting the engine do try to leave room for it.

 

[anderkart]

Does anyone know what the linkage is called that connects to the j arm (shifter) on the engine? Need to make or buy one.

You could buy most any shifter racekart parts you'd need here: http://sharkshifter.com/zc/index.php

 

[walker187]

I bought the cr 80 today for $100. Good thing I got it for such a low price... Opened up the top end to see what the issue was. The guy told me he recently rebuilt the top end but something went wrong and there was no compression. What I found: not only was the ring broken, a piece of it caught on the piston and ruined it as well as the cylinder. So now I have to buy a top end kit and jug and cap. Not really a big deal considering what I paid for it. I will have pics later...

 

[OzFab]

I suspect you'll end up paying more for the parts than you did for the whole bike

Yeah, something went drastically wrong if he's telling the truth (which I suspect he may not be); it takes a lot to shatter a ring like that

 

[walker187]

I suspect you'll end up paying more for the parts than you did for the whole bike

Yeah, something went drastically wrong if he's telling the truth (which I suspect he may not be); it takes a lot to shatter a ring like that

I'm actually going to get the jug and head machined to a bigger bore so I wont spend $200 on new ones. Bigger bore, more power.

 

[walker187]

Machinist said I needed a .050 oversized piston. Anybody know where I could obtain a piston like that?

 

[little billy]

wiseco

 

[little billy]

what year bike

 

[frankysfree]

http://www.diygokarts.com/vb/showthread.php?t=18471

I just posted a vid of an RS125 racing kart on my thread.

 

[frankysfree]

also depending on the engine placement you might need a custom expansion chamber. All the racing karts use a custom exhaust setup to clear you and the frame/body.

 

[walker187]

This bike is a 1987

 

[walker187]

so it's a 48.25mm piston that I need in a .050 overbore.