KTM 250 2 STROKE kart 520/428 chain

[Because Racekart]

Hey guys, me and a buddy just picked up a 2000 ktm 250exc 2s and are in the process of tearing it apart and mounting it to chonda kart he used to race.

Anyway, today was a big day for us as we built and welded the entire motor mount. The next clear step is to find a decent gear ratio and get sprockets.

I was planning on picking up the smallest axle gear i could find (our engine sprocket is a 14 tooth currently and we dont have any room to make it bigger because of contact with the clutch slave cyl.) and just running the chain from the dirt bike. Figure if i could get a 21 tooth axle gear i would be close enough to a 1:1 sprocket ratio.

However, having done more research I've found that the sprockets that people run on shifter karts are 428 pitch, and the ktm's bike chain is a 520 pitch and that these are incompatible.

I have looked just about everywhere and I cant seem to find
a 428 ktm engine sprocket
or
a 520 axle sprocket with few enough teeth for my application.


I'm not sure what to do here, thought i was in the clear and getting sprockets would be the last of my worries. Could really appreciate the help and if anyone is interested I would be glad to post pictures of the project. Also, we are both still 18 and still learning so go easy on us haha.

 

[DS13]

definately post pics. We love em. First off gear ratios should be like 6:1 iirc, with race karts around 3.33:1. 1:1 doesn't seem to like it would work. Secondly, about the chain on the engine sprocket, if ya have a machine shop close by maybe ya could have them build ya a sprocket with the correct pitch and # of teeth. The more experienced guys should pop in here within the next day and help also.

I would hope you have brakes all around with 250 2 stroke and some safety implements implemented. (see what I did there?)

Also

 

[Because Racekart]

Thanks, appreciate the help! I dont know much about gear ratios, but I figured the 250 would have way more torque than I could use, so i had just assumed i would want something near 1:1 to keep it from going through all 5 gears as soon as i hit the throttle. With a 17 tooth engine sprocket do you happen to know what would be an avg axle gear size then? I might get a chance to run this around a small road course when its finished, so i dont mind having alot of top end.

Also, I just started mech engineering at U.T. Austin this year and am on their formula team so i generally have access to their machine shop. A few of the upperclassmen and graduate students have part time jobs machining. I might see if they could build something for me.

On a side note its a single brembo in the rear, but seeing as how much power the bike made, i see adding some front disks as something in the very near future haha

 

[DS13]

Yeah 1:1 isn't going to work. 5th gear is gonna be very sketchy with that 250. Front discs are a must and will be the most expensive part besides the engine itself. With your mechanical background you should know ya get what ya pay for, some things shouldn't be made cheaply.

 

[Because Racekart]

Very true. These are some of the pictures from before we started and after mounting the engine today.

 

[anderkart]

I'd sugest using around a 30-tooth axle sprocket.
#520 sprockets are avalible here: http://www.gokartgalaxy.com/shifter_kart_&_quarter_midget_sprockets_for_3.htm

 

[Because Racekart]

Thanks for the link. So if I kept the 14 teeth engine sprocket, and purchased a 520 30 tooth axle sprocket according to the chart I would be at a 2.142 ratio.

A few questions:

Is this considered a long ratio? about where would mph range would i be looking at?

Also, am I safer staying with the 520 chain? (will be making a chain guard but having a chain break doesnt exactly seem fun or safe regardless)

And also, will i need a new axle sprocket hub?

Thanks again

 

[mckutzy]

Looks good. However it looks like you will need a good heat shield as that exhaust is going to roast your elbow or arm on a right turn. Id say it isn much of a problem on a 4 stroke but this is a dirt bike engine(2stroke, right?).

How will this effect the engine if you have to cut the pipe to change the direction?

 

[Because Racekart]

Thats definitely another thing we've been thinging about. its a 2 stroke. id really like to run the expansion chamber all the way behind the seat parallel to the axle, but idk how this will effect the engines ability to perform.

 

[anderkart]

Thanks for the link. So if I kept the 14 teeth engine sprocket, and purchased a 520 30 tooth axle sprocket according to the chart I would be at a 2.142 ratio.

A few questions:

Is this considered a long ratio? about where would mph range would i be looking at?

Also, am I safer staying with the 520 chain? (will be making a chain guard but having a chain break doesnt exactly seem fun or safe regardless)

And also, will i need a new axle sprocket hub?

Thanks again

Yes you'll need a separate hub for those sprockets. I'd try emailing that site to see if they sell a hub to fit your axle. (what diamiter is your axle, 40mm?)

A high quality #428 chain would probably be stronger that a cheapo brand of #520...

I could give more detailed gearing advice, but first you'll need to research/post your engines redline rpm, primary reduction ratio and try to find all the ratios of each gear in your transmission. (also measure/post the height of your rear tires)

 

[Ted Hamilton]

#428 is lighter and will free up some HP. For shifter kart front sprockets, check out PBI sprockets...they custom hob new ones for shifter kart applications and likely have what you need. For the rear, for #520, go with Brandon Hetrick's sprockets out of PA. They're 7075 instead of 6061 and will last longer (they're what the winged outlaw karts use), or for #428 check out www.sharkshifter.com. Hope that helps...good luck, have fun, and make sure you have good brakes and tires...

 

[Because Racekart]

Hey guys I put in the tire circumference, assumed about a 7500 rpm redline, and the 14-30 sprocket ratio into a calculator and it will theoretically run out of gear around 115-120 mph. Figured this was probably about the right ratio and went ahead and bought a 30 tooth for a 520 chain off of the links that was posted above. I measured the hub and the radius for the inside of the sprocket (where the sprocket sits on the hub)was between 2.25 and 2.5 inches. The bore on the sprocket I bought was 2.373 in according to the site with a 6 bolt pattern (I have a 6 bolt hub currently). Seeing how the only other size available was a 2 inch bore (and that's only for a 4 bolt pattern) I'm fairly confident that this sprocket will bolt right up to the current hub.

Also, we bought the bike with a broken clutch slave cylinder from the original chain coming lose, so the current chain is a brand new upgraded one. (Btw after replacing the slave cylinder we never got the clutch plates from sticking, it's been an unresolved nightmare of bleeding the clutch fluid and opening up the clutch basket) anyway the chain should be plenty strong enough, and we built the motor mount to clamp to the frame so we could tension tension the chain by sliding the engine.

If I ever have to regear the sprockets I might go with a 428 just to try to squeeze out an extra whp but for now i saw the 520 as a safe option that could be applied the easiest and went for it. Dk when it will come in but ill let you guys know how it goes.

Next step I believe will be mounting the radiators. Then shift linkage and figuring a way to run the exhaust. After that well have to face our clutch issues that we've put off.

If anyone knows anything about hydraulic clutches on something like this please let me know.

Also all of the advice on all areas of the build is greatly appreciated!

 

[Because Racekart]

Oh and it's a 40mm axle. Sorry I missed that question. And although I've bought a sprocket already, there's always the liklihood of changing down the road, so ill look for the individual gear ratios.

 

[FatCat]

I'd go with the bigger chain and a high wuality chain. It will have plenty of power. It won't mine spinning a little extra chain mass. Cheap insurance.

 

[Because Racekart]

found this online:
not sure for the year model, but i doubt they changed it much.
14:38
16:36
19:34
21:32
23:30
22:25

 

[Because Racekart]

Oh another thing that I've been worried about if anyone knows the answer. I'm assuming I'm going to need some sort of a water pump for the radiators? Looking at its position on the bike I'm assuming its gravity fed, and I was planning on running it opposite the motor like shifter karts.

 

[fourtogo]

The radiator should pick up the water . The Rad will have some water level above the engine water pump . On my CRF450 a 1/4 of the bottom is below the pump.

 

[Because Racekart]

so the engine has already has a pump mechanism? and as long as some of the water in the radiator is above this pump when we mount it we should have water flow? Also, will this mean decreased flow rates if not all of the water lies above it?

 

[Because Racekart]

Also we just realized another problem. We were planning on using the fuel pump off the kart.

I was thinking it was electric but haven't paid any attention.
Realized in a picture today that there is one fuel line going into the fuel pump from the fuel tank. and then one line going from the fuel pump to the carb. and then one line going from the pump to the top of the old engine. Im assuming this is to create some sort of vacuum to provide fuel suction for the pump. However, Im not sure and i dont know if i can apply this to the new motor.

 

[Ted Hamilton]

Karts utilizing MX engines typically build a "pump around" system for the fuel supply. It's basically:

Tapped hole behind reedcage that a nipple threads into....(you'll have to drill/tap yourself)
1/4" fuel line from above nipple to a diaphragm-style fuel pump pulse fitting
1/4" line from floor tank to fuel pump
1/4" line from fuel pump outlet to base of a "tee" fitting
From the TEE: 1/4" line on left upper of Tee to carby inlet
1/4" line on right of tee (with a restrictor -- see below) back to fuel tank.

For this setup, your floor tank needs (2) fuel outlets, one of which serves as an inlet, AND a vent line. The "restrictor" mentioned in the system can be that screw-on thingy at the top of an old spark plug -- it simply serves to make the fuel "want" to go to the carby instead of back to the tank, but that line's neccesary to prevent overfilling the carby float bowl and pressurizing it.

Hope that makes sense...drop me a line if not. This allows you to replace a gravity-fed bike tank with the floor tank typical to a kart....