03 YZ250f carb adjustment?

I don't have a manual for this bike yet and am new to 4 strokes so I have a few carb questions. Bike ran great last summer, but header on yoshi pipe cracked so I put the stock back on. That was the only change made to the bike over winter, other than that it sat and now idles and runs really rough. On my 2 strokes the carb adjustment is just idle screw/air screw this seems to be a bit more complex, can anybody fill me in what screws are for adjustment and what kind of adjustment do they make?

I'm hoping it doesn't need to be re jetted because of the pipe change

Go to the dealer and buy a brand new pilot jet. Do not bother to clean the old one. Then fine adjust the fuel screw for a perfect hot idle. The fuel screw is on the bottom of the carb.

In the forum FAQs, is informatin about D/L a Factory manual direct from Yamaha, for free.

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

Thanks william. Added some sea foam to the tank of gas and rode the bike around my friends yard for a bit and it seemed to get better and better. Its actually running smooth enough to take to the track with me tomorrow! Carb isn't dialed in perfect but its running good enough. Good to know thats the fuel screw, it looks like to adjust it you have to tilt the carb (top side toward the left side of the bike). I found that strange at first cause on a 2 stroke you just tilt the float up to the left side of the bike, but there is a part/cable that hits the frame on the 250f. I got a better idle by adjusting the black knob around the middle of the carburetor. I thought this was the throttle positioning sensor adjustment but it seemed to affect the idle.

The black knob sets idle speed. Do not touch the TPS unless you follow the service manual procedres for adjusting.

Invest in an extended fuel screw. Only consider stainless steel or brass. Do not by an alloy/anodized one.

Tilting the carb, tweaks the fuel level and you'll never get the fuel screw set correctly.

The black knob sets idle speed. Do not touch the TPS unless you follow the service manual procedres for adjusting.

Invest in an extended fuel screw. Only consider stainless steel or brass. Do not by an alloy/anodized one.

Tilting the carb, tweaks the fuel level and you'll never get the fuel screw set correctly.

What's wrong with the anodized screws?

What's wrong with the anodized screws?

A couple of things.

Aluminum is very soft, the tip of the screws wear and break off very easily

Galvanic action occurs between the alloy and the brass seat, damaging the seat, sometimes seizing the tip and braking it off

Often, the tip is not a round even tip, it is a bit conical. This potentially causes the tip to get wedged in and break off and it causes fuel screw function to be off. This results in being very difficult to get it set and sometimes requiring an abnormal reduction in pilot jet size (like going from a 45 to as small as a 38).

More than enough reasons for me to never use one.

BTW, the link you provided is all encompassing and very vague and in some places, incorrect and/or out of date. It should only be considered to provide the broad strokes/concepts and certainly not a guide.

Well I rode the bike quite a bit yesterday at the track, ran really well! It does pop on decel though so I believe thats a lean condition? Also, when comparing the idle to other 4-strokes, it seems a little less smooth. I wouldn't say its rough, but it doesn't sound 100% perfect. I'll try and take a video to give a better description.

So the fuel screw on the bottom, without an aftermarket screw, whats the correct way to adjust it? It seemed silly to me to tilt the carb for each adjustment, it would take an hour just to get the screw set right. Do I need one of those screwdrivers with a 90 deg bend?

Get a stainless or brass extedned fuel screw.

A couple of things.

Aluminum is very soft, the tip of the screws wear and break off very easily

Galvanic action occurs between the alloy and the brass seat, damaging the seat, sometimes seizing the tip and braking it off

Often, the tip is not a round even tip, it is a bit conical. This potentially causes the tip to get wedged in and break off and it causes fuel screw function to be off. This results in being very difficult to get it set and sometimes requiring an abnormal reduction in pilot jet size (like going from a 45 to as small as a 38).

More than enough reasons for me to never use one.

BTW, the link you provided is all encompassing and very vague and in some places, incorrect and/or out of date. It should only be considered to provide the broad strokes/concepts and certainly not a guide.

Listen to this man. I broke the tip off of an aluminum screw and could not extract it. My only choice was a new FCR carburetor as the mid body cannot be purchased separately. Luckily I found a used one on fleabay for 125.00.

I replaced the fuel screw w/ a JD jetting screw which has a brass tip. Adjusting the fuel screw with an improvised tool is a PITA.

As a temporary measure I found that a small flat screwdriver bit from my drill kit fitted nicely into the hole for minor adjustments. But an aftermarket screw is cheap and easy.

so I got an aftermarket screw today. Is it as simple as tilting the carb, removing the stock screw (i know theres springs and washers), and installing my new screw and making the carb adjustment?

Yes. If your pilot is correct size it should be a 30 second install and then a 5 min warm up to do final adjustment.

Grrr...I hate how much is going on inside that frame hard to work on anything! The carb won't tilt either way, too much stuff in the way blocking it, so I have to pull off the subframe and get it off that way.

Work from the shifter side. Tilt the carb a little bit towards you and remove the hot start.

Once the hot start is out, it's pretty easy to tilt the top of the carb away from you.

You don't really need to remove anything other than the hot start.

Work from the shifter side. Tilt the carb a little bit towards you and remove the hot start.

Once the hot start is out, it's pretty easy to tilt the top of the carb away from you.

You don't really need to remove anything other than the hot start.

I would just take it off every time, I've had bad experiences with tilting, only because I had to do it so much, but watch the front rubber mount to the engine I've broken those little flanges that hold it in position and thus caused a minor air leak.

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