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Old October 23rd, 2013, 10:47 PM   #1
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Name: David
Location: Portland, OR
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Motorcycle(s): 09 Ninja 250

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Main jet question

When I have seen videos of carbs being cleaned it seems the main jet usually has several holes on each side. Seems it is usually 3 evenly spaced down each side. When I had mine out there was only 1 hold on each side.

I cannot find a picture of a stock jet for the life of me. I don't think the carbs had ever been off the bike before I took them off to clean them, so it is probably correct. Just wondering.

Thank you.

It's on the side, but I forgot to mention my bike is an 09 ninja 250.
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Old October 24th, 2013, 05:15 AM   #2
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Those holes are for mixing air and fuel and that is an emulsion tube, which is in between and also holds the main jet and the needle jet.

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"In the real world, the air demand created by an engine is anything but steady state, even in a V-8. Because of this, the air-corrected main jet may still not supply the desired ratio of air to fuel at all points of operation. To compensate for this, an ingenious system was devised called the emulsion tube. On an engine employing a single barrel of carburetion connected to each cylinder, the emulsion tube design is critical for the effective function of the carb. As more cylinders are connected to the carb, the airflow gets closer to steady-state, and the emulsion tube's function as a curve-trimming device becomes less critical........ The good news is you are about to get the PHR method for simple emulsion tube "reading."

Calibration Considerations
The calibration components considered so far have been the main jet, the emulsion tube, and the air bleed. Starting at the main jet, we find that a larger main jet makes the mixture richer, and vice versa. The effect of the emulsion tube will depend on the hole pattern. Here is how to read it: First, hold the emulsion tube upside down and inspect the hole pattern. Holes at the top of the emulsion tube will affect the top-end of the rev range. Holes in the middle will trim the mid-rpm range, and holes at the bottom, the low-rpm range. Where there are no holes, the mixture will be rich. Where there are holes, the mixture will be leaned out. Just how much the mixture is leaned out by the presence of holes depends on how many, and how big. The more holes present, the more the mixture is leaned out at that point. Because it is fed with air from the air bleeds, the emulsion tube's overall function is influenced by the air bleed size. A larger air bleed leans out the mixture, but at low rpm and small throttle openings, the air bleed has little influence over the mixture. As the engine's demand for air increases due to an increase in throttle opening and rpm, so the air bleed's influence increases. At high rpm, just a few thousandths change in the air bleed diameter can have a significant effect on mixture.

One other aspect of the emulsion tube and well is that they act not only as a means of calibration but also as a control element for fuel atomization. By emulsifying the fuel prior to it reaching the booster, the fuel is easier to shear into fine droplets at the point of discharge. Generally, the more it is emulsified with air in the emulsion tube, the easier it is to atomize at the venturi. With an understanding of how it is achieved, let us now look at what we need the main circuit to deliver in the way of air-to-fuel ratio."

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"The Main Circuit’s ultimate components are directly below the needle jet and includes the midrange system (above) PLUS the main jet, emulsion tube (between the main jet and the needle jet) and the main-air correction jet (in the perimeter of the carb’s “mouth”, opposite the pilot air correction jet). The function of the main jet is to limit the total amount of fuel available through the carb, at wide-open throttle. The main air correction jet admits air to the main system, through a channel that connects to the emulsion tube directly above the main jet, and that air also acts as a fuel/air ratio modifier and emulsion improver."
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