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YZ250F 2003 Stalling Problem


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Your going to have to give us a little help........

Stock Jetting???????

Altitude????????

Temp???????????

I'm just guessing here, but look into BK mod, could be your accelerator pump. The BK mod will allow you to reduce the spray duration of your accelerator pump, too long is too much, and will cause a bog off idle when you wack it............

Dodger :D🙂

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well check the adjustment for the accellerator pump. That would be my first suspect.

You didnt say, but I assume it has done this since you got it right?

For reference: I went 1 larger on the main jet, and raised the needle one notch. It could still go up on the needle. But I am gonna test out an 02's needle first.

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There is a ton of jetting information that has been posted on this site.

For your problem bogging on bottom can often be adjusted out by leaning fuel screw or going to smaller pilot.

Here is something taken from a similar post on the 426 side. It applies to your bike as well.

==========================================================

A good jetting post is on the YZ250 side called "Jetting 101". Use the search function and read it.

You need to understand that there are basically three jetting circuits:

1. pilot/fuel screw for low throttle openings,

2. needle for mid throttle openings,

3. main for mid to WOT

and that they all overlap. So you need to jet each throttle position.

You also need to know that the stock jetting on the '01 is decent for sea level at about 70-75 degrees. (Maybe a tad rich on the main and needle but good enough to have loads of fun.) You typically need fuel screw adjustments on a daily basis to keep the pilot circuit dialed - depending on temperature.

Then, use the following to help you dial-in from stock

1. Jet leaner for higher elevation (edited: I originally said richer which is flat out WRONG! Thanks Hick)

2. Jet leaner for higher temperature

3. Jet richer for lower temperature (although you can get away with stock except for pilot/fuel screw)

Here is how to get richer or leaner

1. Fuel screw OUT = richer at low throttle/idle

2. Pilot jet bigger = richer at low throttle/idle

3. Needle clip position lower = richer at mid throttle

4. Main jet bigger = richer at mid-WOT throttle

Also

1. popping on decel is a lean pilot

2. popping anywhere else is also probably too lean

3. blubbering or sputtering is usually rich

4. wheezing or hesitation is usually lean

5. hanging idle is usually lean pilot circuit (or plugged PAJ)

6. hard starting when cold is usually too lean (blip throttle a couple of times or turn fuel screw)

7. hard starting when hot is usually too rich (use hot start button)

8. bogging on bottom can often be adjusted out by leaning fuel screw or going to smaller pilot. Some bikes are candidates for the BK mod (see motoman393's site - link on TT home page)

That's about as simple as I can make it!

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8. bogging on bottom can often be adjusted out by leaning fuel screw or going to smaller pilot. Some bikes are candidates for the BK mod

Maybe you should have included more info on the accelerator pump other than just some bikes are candidates for the BK mod. None of the 02's really needed that mod and he's talking about an 03.
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I dont think you have a problem at all.

to me it sounds like you're going from shut to full open in no time, that causes stalling anytime with all four strokes.

You have to roll off your throttle.

correct me if i'm wrong.

greetz

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Maybe you should have included more info on the accelerator pump other than just some bikes are candidates for the BK mod. None of the 02's really needed that mod and he's talking about an 03.

Noted. I wanted to keep it as simple as possible.

As for as who "needed" the BK mod - I for one have never done it to my '01 426 and I still grin from ear-to-ear every time I ride. I think the people who gain the most from it are the owners of '00 426's. Although others with newer 426's swear by it. I have no off-idle bog or hesitation and don't feel I need it. However, I rarely "whack" the throttle wide open from closed at low revs. I tend to ride with the revs up and roll-on the throttle in most situations. It could be a riding style thing. 🙂

Anyone who is interested in knowing more about the AP can do a search for "Accelerator Pump 101" and "Jetting 101" on this side. I believe they were posted around May so you would have to use "within the last year" on the search page. They are both excellent and very detailed posts.

Steve T

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