YZ 250 carb question.

mxracer_0141

PR Member
so i have a 2012 yz250 2 stroke. about 3 months old, rode it probably about 10-15 times at the very most. im having trouble with it in the midrange power with it cutting out, spitting and sputtering about quarter to half throttle. like if i wanna hold it half throttle and coast it in that speed up to a jump it spits and sputters real bad. in the bottom end and top end its strong as hell. i took the carb apart and dropped the main and pilot down one (stock 178 main, 50 pilot) down to 175 main and 48 pilot. didnt do anything, actually made it a little worse. i run a 36:1 mix with amsoil dominator pre mix with 110 octane gas. i read in my manual that i have to move my clip down one? im not a guru on jetting so im kind of in a rut on where to go with this. does anyone with a newer yz250 have any carb setting suggestions for me to have all around power throughout my whole throttle range? if anyone can help me id greatly appreciate it!
 
If it is spitting and sputtering it is too rich. The first thing you need to do is empty the gas tank and replace it with a 50/50 mix with your race fuel and 93 octane. Your bike isnt designed to run on straight race fuel in stock form. If after changing the fuel you still experience the same problem then raise the clip on your needle. This will lower the needle in the needle jet and lean it out. Only move the clip one position. The reason for changing the needle is because it effects the mid range ... half throttle area.

Keep us posted
 
Check the reed valve pedals. Sounds like you may have frayed edges or a broken one. <O:p</O:p
Also, check for air leaks......around the carb boot, reed spacer and reed block. <O:p</O:p
Check power valve function.....all three valves. They need to work in unison or they will bind up on each other.
 
Ihave a 08, its the same bike...i am running the stock main and pilot with the next leaner needle in the second clip... i think it may be a n3cw or somthing...but like hershey said the bike will run bad on full race gas... i mix 2 gallon of race fuel to 3 gallons of 93 at 40:1...
 
ill try all these when i tear it apart tomorrow. sharc, with it being a new bike i just cant see any of those problems happening. like i said its only been rode at the most 15 times (i dont even think that honestly) but ill double check it all just to be safe. should i put the stock jets back in it (178 main, 50 pilot) and move the clip or leave the jets that i have in it (175 main, 48 pilot) and move the clip? also with the mixture 40:1 wont put me in danger of seizure will it? ive always ran 36:1 ever since ive been on 80s even my 05 rm250 ran perfect on 36:1. just a little iffy on that mixture because ive heard guys with 250s running 32:1 and having no problems with richness. thanks for all the input guys, ill take all this and hopefully can figure it out because its frustrating having a brand new bike the runs like garbage!!!!
 
If your running straight 110 that may be your problem there. Unless you have cylinder mod's you shouldn't be running it like that.
 
Oh, I was assuming it ran fine at first and then started to run poorly. Has it run poorly all along?
If so, go back to stock and start with the gas suggestions. You don’t need that high of octane in a stock motor.
Also, hunt me down at AC or Smith and you can take mine for a spin for comparison.
 
Im sure it probably has but when i got it it was winter and rode ram jam which you cant tell if your having midrange problems in there, first time i had it on an outdoor track was at TV land and noticed it bad, then went to AC and just got worse. Might just be that 110 like you guys have said. Ill have to catch you on weekends because i work second shift now 3-11 so my riding during the week has vanished. sucks!
 
Along with everything previously recommended, make sure your running the spark plug that is recommended for that bike. Simply using one without an "R" when one is needed can cause the sputtering you described.
 
My uncle was having the same problem and he put a pro circuit pipe on the bike and the problem went away. Maybe a fluke thing. Do you have a pipe?
 
no its all bone stock, i raised the clip up one to the first groove, put the stock main and pilot in it and put 93 mixed 40:1 and it cleared it up big time, it still pops here and there on the mid range a little bit anything i can do with the jetting now to perfect it? stock main 180, stock pilot 50.
 
try the next leaner needle... my 08 came with it... its a half step leaner in each clip... running mine in #2 position..
 
Sound like you are on the right track.
Give the carb a good cleaning in all the jets: idle, air, main.
Be careful not to get too lean trying to compensate for something else. Especially with warmer weather on its way. You could melt a hole in the piston or head if you get too lean. A can of carb cleaner and an hour of cleaning is relatively inexpensive and will never hurt.
 
yeah i gave the carb a good cleaning when i had it all apart friday. right now im running stock jets with the stock needle with the clip in the #1 position (it came in the #2 position stock) i mean its hard to tell as of now because like you said TC, warm weather hasnt hit us yet so that may be the issue as well and i totally agree with you, i personally like to run my bikes more rich than lean. im just curious if it has anything to do with the air screw now?
 
So hard to pin down sometimes, just be careful. Sounds like you are aware of what you are doing.
Maybe just a good aftermarket pipe will perk it up. Maybe even gearing down a tooth on front or up one or two on the back to suit your riding style if you tend to lug the motor may be just the trick.
You could just ride with the throttle locked wide open and use the clutch as a governor for power delivery and not worry about any jetting!!!
 
Try the pipe. If you're at a track I'm at we can put mine on yours and test it for a few laps....then you'll know for sure.



Hey have you checked your compression? (Unrelated to Carb) But, my buddy bought a 12 yz 250 and he's got 225 pounds of PSI when measuring with the spark plug guage (Screwed into plug hole on head). For some reason I think you're only supposed to have 140...we got to talking and thought maybe it could be a manufacture defect....
 
I've been fighting the same thing. I have found a lot of info out there on the issue. There has been a pretty well documented thread on Thumpertalk dealing with the timing. It is thought that alot of these bikes came out too far advanced. They also have really good info on setting the timing to "adjust" the power delivery to your liking. As far as the carb goes there are some good tips out there for that set up also. The one that really cleaned my mid range bobble out the most was going to the #8 slide. It will also smooth out idle and make air screw adjustments less sensitive. The jetting itself I am at a 45 pilot, 175 main. N3EJ needle in the 3rd position. then the air screw at 1 1/2- 2 turns out. I am running pump gas (93 octane) and Motul 800 at 40:1. Mine is running very good at these settings. I am gettting ready to do more experimenting with the timing just to see how much of an impact those adjustments can make. Hope this helps.
 
That's a GREAT article.

That being said....alright Cameron, start Weldin a Ktm engine in my yz250 frame. Name your price.
 
Back
Top