gearing on yz250

Scott Racing

New member
I just put a yz250 on kart. Running factory tranny and run 1/8 to 1/10 mike tracks. I have a 14 27 gear on it now. I'd like to keep it around 9000 and figured you guys could give me some advice.
 

alvin l nunley

Premium User
I just put a yz250 on kart. Running factory tranny and run 1/8 to 1/10 mike tracks. I have a 14 27 gear on it now. I'd like to keep it around 9000 and figured you guys could give me some advice.
Did you know that not all shifters have the same top gear ratio to the output shaft? Of all the shifters I have in my gear ratio chart for shifters, only the Honda is 1-1 on the final ratio. This means that if yours is not 1-1, and someone gives you a gear set, it might not be the same ratio, overall, that you would need. Post your output ratio and if someone gives you a ratio, I’ll check it in my spreadsheet and tell you what gear set you will need to end up with the same overall ratio.
Comments, compliments, criticisms and questions always welcome.
 

Ted Hamilton

Design Drafter / Racer
Keep it around 9000?! I'd think any 2-stroke 250 would like to sing, closer to 12-14,000 rpm...Why 9000?
 
Actually Ted a lot of the 250 moto engines put out peak torque/horsepower at lower rpm's than we are used to turning with kart engines. They weren't designed around centrifugal clutches and things of such as we use in karting. I'm not saying they "can't" but..... :)

The air cooled YZ on my vintage Dino shifter pulls best between 8500 and 10,000 with my homemade pipe (variant of a Jardine). With that, he may not be too far off. It's really gonna take some track testing to determine what is best. JMHO -Alan-
 

Taft Racing

Member
It's mostly in the pipe, you can build pipes that will turn allot more rpm than the 9,000. the transfer ports on the 250's most all of them just start making power at 10,000 most are way to big for the lower rpm range. they restrict the exhaust ports to bring the torque up and thats what keeps the hp and rpms lower. Your MX pipes work in a narrow range, they are all designed to shift. Make max torque/hp for a short 2-3000 rpm band then you kick another gear. the pipe will shut off over 9,000

If you buy a kart specific pipe QRC or others becoming available you can twist them up over 11,000 especially the earlier YZ 250's they have a much shorter stroke than most all the other manufacturers. 97 up most every one went to the long 72mm stroke. Piston velocity will keep those rpm ranges down. If you want to turn the rpm range up open the exhaust raise the port height some and get a kart pipe. then hang on lol.
 

Scott Racing

New member
Thanks for responses guys. I tested it today but IM gonna have to drop a couple teeth on rear. I'll look into those pipes Tim Thanks
 

arc100

Member
Careful turning 12500 with a 72mm stroke. If you're not making power turning the revs is just killing the engine. Also when it comes to mx engines, remember they have been designed for a purpose, the ignition curve changes drastically at high rpm because the designers assume that if the engine is turning above normal peak rpm then the wheel must be in the air so as a safety for engine damage from the over-rev they cut back the ignition. If your really interested call Vortex ignitions and they can send you a normal ignition curve from a mx engine. PVL also can help with this info. Easiest solution is just use a kart ignition. But still careful with the rpm.
 

Scott Racing

New member
Sounds like 9000 OS a good number for me. It makes max HP 8800 max torque 7600. Unless I change Alot on engine it sounds like 9000 is max
 

bh1974

Member
Just my opinion but start higher overall ratio, motors make more than enough power to pull a kart that's under geared. Mines still work in progress, and its NOT EASY to drive on a bullring. Caneyville? Last time we were there, all we heard was *****ing and moaning cause we ran 2cy les.
 

Taft Racing

Member
Sounds like 9000 OS a good number for me. It makes max HP 8800 max torque 7600. Unless I change Alot on engine it sounds like 9000 is max

Stock MX pipe stock MX ign stock MX porting yes your in that window. alter any of those and the rpm max goes way up.

I built a converted mx engine altered the exhaust some and made a kart pipe, the engine came alive at 10,000 It's a work in progress. better kart related pipes and porting. But for sure your going to have allot of fun when you hit the throttle.
 
Talked with a 250 mini sprint builder that has his pipe and says it will not turn over 9500rpm and really falls off at 9000rpm. He told me he would love to sell me one but doubts a kart could ever hook it up as it a very peaky pipe. Food for thought.
 

bh1974

Member
4th would be better, makes for a better primary ratio. Sure somebody with experience and success using one with the transmission could answer better
 

alvin l nunley

Premium User
Is there a certain gear you should shoot for. Should I try to gear it to run in 3rd or 4th or does it matter.
Seems to me that the gear ratio from the crank to the axle is the important thing. Whether you use 3rd or 4th, the overall ratio is what you want to know.
I’m sure you know that all shifters can/do have different ratios in 1st – 5/6th. The only way to know if your gear ratio is the same as someone else’s, if they have a different engine, is to know the ratio from the crank to the axle. My spreadsheet makes this a real easy problem to solve.
Some examples;
Honda CR125 5/6th 1-1 X 13/26 = 2.42 crank to axle.
Pavesi 125 ICC 5/6th .778-1 X 13/26 = 1.56 crank to axle
Big difference you must agree.
But if you gear the Pavesi 12/38 you come up with 2.46 from the crank to the axle. Not exact, but close. And this was a 2 minute calculation with the spreadsheet.
You must agree you need to know the ratio of the gear set you choose to run to come close to your final ratio from the engine to the axle. If someone gives you gear set numbers, and they have a different engine from you, they may or may not be close to what you need to achieve the same crank to axle ratio.
Comments, compliments, criticisms and questions always welcome.
 

alvin l nunley

Premium User
I find it strange that there has been no discussion on my post about the final gear ratios.
Now the Honda, in most cases, (some Hondas are not 1-1 in high gear) is 1 to 1 to the output shaft, so you don’t have to worry about transmission ratios, but you do with all the other engines on the market. If someone with a Honda gives you a gear ratio, and your engine is a Pavesi or TM etc., that Honda gear ratio is going to do you absolutely no good. You really do need to know the ratio for high gear in your engine.
Comments, compliments, criticisms and questions always welcome.
 
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