Main Jet Removal

Quick question for the group. I may need to change my main jet for a ride coming up. The reason is a 2000-3000ft ALT diff. Can the main be changed without removing the whole carborator? or what can you change by removing the bowl pilot, clip pos, main, nothing.


You can change the main jet without removing the carb. Drain the bowl of gas by opening the allen screw on the side of the carb. The gas will drain out of the drain hose. Take a 17mm wrench take off the large nut on the bottom of the bowl. The main is right there, you may want to use a mechanics mirror to locate the main, I just feel for it. You need a 6mm to remove it.

Through the hole wylie1 is referring to, the main jet is the 6mm jet in the middle. The pilot jet is the flat head screwdriver recessed in a hole toward the front of the bike, and the starter jet (I've never heard of anyone touching it) is behind and slightly to the side of the main.

If you go ahead and take the bowl off (you can take it off easily, well almost easily with removing carb-need a short 3mm allen wrench), you will see the float and could possibly get to the inlet needle (not jetting needle) but the only other thing you get to is the leak jet. It is on the bottom of the bowl and takes a very fine flat screwdriver.

To change the needle or clip position, you still don't have to remove the carb. Just remove seat and tank, with a long 3mm allen with a cupped head, go through the frame in the back and behind the wiring harness in the front and remove lid from carb. Twist throttle once to move needle up (it comes up with the slide) and use needle nose or tweezers to get it out.

Good luck,


Are you saying the needle comes out through the top of the carb or the bottom. I looked in the manual but no pictures on the subject. Thanks for all the input, wanted to know if this was going to be a couple of hours deal or a few minutes. My bike runs great right now but don't want to get to Utah and have the thing start fouling plugs.

Thanks again

You can change all three (pilot, main, and needle clip position) in less than 15 minutes.

The needle comes out through the top. You need a 3 mm allen for the top of the carb. Once that is off, the needle is underneath another larger allen screw/spring in the slide. You don't have to twist the throttle to get it, just use tweezers. I usually loosen the carb boots and twist the carb accordingly (top to the shifter side to get the needle, bottom to the shifter side to get the pilot jet, main jet.

The altitude change of 3000ft is about equivalent to a 30 deg (F) temp change.

Wylie was spot on about using the mirror. As mentioned, drain the fuel and then remove the bottom plug on the bowl with a 17mm wrench. Using the mirror, look up inside and you will easily see the 6mm brass main jet(hex head). It is located off-center toward the airbox side. The pilot requires a narrow flat blade screwdriver and is recessed deeper and toward the engine side of the hole.

I can change the main jet in less than 5 minutes without twisting the carb. It is SO easy. The pilot is a little trickier since it is much smaller and it's not easy getting the screwdriver blade into position unless you use the mirror. BTW, if you do R&R these jets, make sure you are gentle when reinstalling them. Brass threads are easy to strip. Just snug them down with a little pressure...and then just a hair more.

[ May 10, 2002: Message edited by: Boit ]


On the 250F forum, there is a 'Jetting 101'. It is only the most basic beginner tutorial. Might read it.


Man I didn't know I wasn't suppose to ask basic questions on this forum. Just thought I would ask the guys that had the same bike as I had...Seemed to make sense at the time. Sorry to waste your time professor. :)

Whoa, ranger. I didn't mean it at all that way. I was just saying that jetting 101 wasn't esoteric aspects of jetting. It tells how to get to various parts of the carb and just the basics of jetting. Just what I thought you might need, and in more detail that we are providing here.

I was trying to make it more approachable, not saying you weren't up to understanding something more advanced. I'm truly sorry I sent that message.



It sounds as if you feel you were told not to ask basic questions. I don't read that in any of the replies. Most of us here have been in the same spot at one time or another and are more than willing to help. Well, maybe everyone except EgoAhole anyway. :)

BTW - As said earlier, "Jetting 101" on the YZ250F board is a very nice write up about the carb. It is definitely recommended reading.

OK Fellas,

I wasn't really pissed off, but that last one kinda sent the message that I can't manipulate a search on a computer. I know you guys give good info but when I'm told to go to a forum "not even the same bike", "same carborator?" what the heck.

I have seen numourus posts saying the carb was worth $750 bones, I for 1 don't have the cash to get it back up if I ***k it up hence the original post.

I do appericiate the input :)

ranger, hey troop, did you get a manual with that bike? if not i'd get one pronto, it has helped me alot with my 426. btw, here in IL it's starting to feel like wash st, i think i'm gonna have to get out the mud tires. good luck on the high ride.

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