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Stupid Question - Size of pilot Jet & operation of the fuel screw

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Hello Chaps,

Sorry, I have a pretty basic question that you guys might know.

On a YZ250F 2007

Fuel Screw - By turning it clockwise, I increase the amount of air? I'm assuming that basis the fact that that if less than 1 turn out, I should reduce the Pilot jet, so the amount of fuel should reduce. Correct?

Needle jet - I removed my needle jet and all it's written on it is C32. Is this an indication of size?? :smashpc:

Tks

Edited by danilopucci
Typo

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Fuel Screw/Pilot Jet

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 1,800 rpm, but go by the spec in your manual.

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Hello Chaps,

Sorry, I have a pretty basic question that you guys might know.

On a YZ250F 2007

Fuel Screw - By turning it clockwise, I increase the amount of air? I'm assuming that basis the fact that that if less than 1 turn out, I should reduce the Pilot jet, so the amount of fuel should reduce. Correct?

Needle jet - I removed my pilot jet and all it's written on it is C32. Is this an indication of size?? :smashpc:

Tks

The fuel screw interrupts the pilot jet circuit allowing more or less fuel.

You say needle jet then you say you removed the pilot jet. Confusing.

Needle jet (jet needle) does not work with the pilot net; it works with the main jet.

A needle jet is the tube the jet needle drops into. The 'needle' is called a jet needle. The 'tube' is called a 'needle jet'.

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The fuel screw interrupts the pilot jet circuit allowing more or less fuel.

You say needle jet then you say you removed the pilot jet. Confusing.

Needle jet (jet needle) does not work with the pilot net; it works with the main jet.

A needle jet is the tube the jet needle drops into. The 'needle' is called a jet needle. The 'tube' is called a 'needle jet'.

Apologies the confusion. Read nedle jet where it pilot jet. I edited it

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William1

I appreciate the lenghthy explanation, but the question is more to understand the physics and engineering behind the carb.

In short, by turning the fuel screw clockwise we add air or remove air?

1. Add air

2. Remove air

3. Add fuel

4. Remove fuel

5. None of the above ?!?!

Tks

Danilo

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Fuel screw and pilot jet are two, seperate but related circuits. Both supply fuel at idle (all the time actually but they are the only two items that supply fuel at idle).

Pilot jet is considered the coarse adjustment, fuel screw is considered a fine adjustment. The full range of the fuel screw ias approximately equal to a pilot jet size.

Air is controlled by the idle speed screw.

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William1

I appreciate the lenghthy explanation, but the question is more to understand the physics and engineering behind the carb.

In short, by turning the fuel screw clockwise we add air or remove air?

1. Add air

2. Remove air

3. Add fuel

4. Remove fuel

5. None of the above ?!?!

Tks

Danilo

1 No

2 No

3 No

4 Yes

5 No

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The fuel screw on a 4 stroke adds or removes fuels not air. Clockwise and you are reducing the amount of fuel, counter clockwise and you are richening it up. I think you are confusing it with the air screw that 2 strokes have that add or remove air from the circuit.

Once you understand that then the tuning procedure in post #2 will make sense.

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The fuel screw on a 4 stroke adds or removes fuels not air. Clockwise and you are reducing the amount of fuel, counter clockwise and you are richening it up. I think you are confusing it with the air screw that 2 strokes have that add or remove air from the circuit.

Once you understand that then the tuning procedure in post #2 will make sense.

You nailed it!

Thanks guys

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