Check the O-ring at the Back of the Screw Hole After Removing the Pilot Screw Which is Often Overlooked

The air screw, which increases or decreases the amount of air before it enters the carburetor, or the pilot screw, which increases or decreases the amount of air mixture after the air and fuel are mixed, are used to adjust the carburetor at idling. Although these parts are often thought to be irrelevant when driving anyway and are not considered, they are surprisingly important and should be taken care of during maintenance.

Pilot screw easily intake secondary air

The OEM carburetor for Kawasaki Z1000J is a pilot screw type, and the screw is located on the engine side from the vacuum piston. If you are not sure whether your motorcycle's carburetor is an air screw or pilot screw, a certain criterion is whether the screw is on the air cleaner box side or the engine side, bordering the throttle valve (vacuum piston).

The O-ring of the pilot screw of this carburetor is built into the base, so there is no need to worry about forgetting to remove it compared to the screw that is set on the tip side above the spring. To find out what kind of structure your motorcycle's carburetor has, refer to the parts list in advance, or carefully observe the actual parts when disassembling before proceeding with the work.

The air screw and pilot screw are important parts for the carburetor to adjust the mixture ratio at idle with the throttle fully closed. The service manual specifies a standard return rpm, but in reality, turning these screws slightly after determining the idling rpm with the throttle stop screw will raise or lower the engine rpm. The best return rpm is determined by adjusting the throttle stop screw again to match the point where the rpm increases.

Air screws and pilot screws are greatly affected near idling because at the time of idling when the throttle valve is closed, the venturi, the main passageway for air, is blocked and the negative pressure on the pilot system passageway is greatest.

The air screw and pilot screw are the same in that they are both adjustment mechanisms at idle, but they differ in the specific way they work. The air screw is located on the air cleaner side of the throttle valve and adjusts the flow of air before it mixes with fuel in the float chamber. The pilot screw, on the other hand, is located on the engine side of the throttle valve and adjusts the volume of the air mixture after the air and fuel have been mixed.

Both screws have tapered tips, and loosening (returning) the screws from the lightly tightened position expands the area of the passage, increasing the amount of air suctioned in the case of the air screw and the air mixture in the case of the pilot screw. At this point, the difference in the way both screws work becomes apparent. When the air screw is loosened, more air is suctioned into the carburetor and the mixture becomes thinner, whereas when the pilot screw is loosened, the absolute amount of air mixture flowing into the engine increases and the mixture becomes thicker.

Specifically, if the mixture is felt to be too thin at idle, motorcycles equipped with airscrews tighten the screws to squeeze out more air to increase the fuel ratio, while motorcycles equipped with pilot screws loosen the screws to increase the fuel mixture. In other words, the screw adjustment is exactly the opposite.

There are also differences between air screws and pilot screws in the parts associated with the screws. Both are not tightened but returned to an arbitrary position, so a spring is incorporated to prevent turning by vibration of the engine or carburetor itself, and an O-ring is used with the pilot screw. The reason for this is the position of the pilot screw.

As mentioned earlier, the pilot screw and the pilot port, which is the outlet of the air mixture, are located on the engine side of the throttle valve, and a large negative pressure is applied when the throttle is fully closed. A large negative pressure means that air is suctioned in from wherever it can be suctioned in and whenever there is a chance. Even a small gap between the male pilot screw and the female thread on the carburetor side can become a pathway for air, and the O-ring that fills the gap between the two is incorporated to prevent this.

Also, while only air passes through the air screw, the mixture that passes through the pilot screw contains fuel, and the O-ring prevents fuel from seeping out of the carburetor through the gap between the threads.

Although we do not know how much difference or difference in the pilot screw return speed will be caused by the presence or absence of the O-ring, there is no reason to remove the O-ring at one's own discretion from the specifications that the manufacturer decided to incorporate during development to ensure the pilot screw's airtightness. O-rings that have hardened over time can also cause a loss of airtightness, so we recommend replacing them with new ones on the occasion of overhauling the carburetor of the discontinued motorcycle.

When removing the screw, do not forget to remove the O-ring at the tip.

A pick tool that can remove miniscule O-rings. Although it is not dedicated to the pilot screw, it is extremely effective when overhauling carburetors. Regardless of whether you are overhauling the air or fuel passages, it is essential to be careful not to damage the delicate interior of the carburetor.

The pick tool's earpick-like tip is the perfect shape for removing O-rings and washers left in the carburetor. The trick is to insert it into the hole where the pilot screw was removed and gently hook it and pull it up. The O-rings removed are crushed and deformed due to age-related deterioration, so they should be replaced with new ones when the screw is restored.

When removing the pilot screw, it is common to forget to place the O-ring and washer. If you understand the difference between an air screw and a pilot screw as explained earlier, you can imagine that the O-ring and washer are still in the carburetor, even if only the spring comes out when the screw is removed. If you proceed, it may lead to trouble.

One example is crushing O-rings and washers. When the pilot screw is removed, the O-ring or washer remains on the carburetor body at an angle, and without realizing it, the screw is tightened and pinched. Since the pilot screw is tightened once and then backed out for the specified number of revolutions, crushing the pilot screw when tightening it is the worst case. If the washer is not in contact with the washer, it may damage the tapered part at the tip of the screw, and then the air mixture cannot be adjusted properly.

Double assembly of O-rings is another common failure. The screw, spring, and washer came off as a set, but the O-ring did not come out, so the operator decided that maybe I dropped the O-ring somewhere in the momentum of removing the screw, or maybe the previous operator forgot to put it on. A new O-ring is put on the screw and tightened even though it is still inside the body.

Both of these mistakes can be prevented if the parts that came off together with the screws and the parts remaining on the carburetor side are checked carefully and carefully. However, it is also true that the O-rings and washers that remain inside the carburetor are difficult to check because they are deep inside the screw holes. However, it is also true that O-rings and washers left inside the carburetor are difficult to check because they are deep inside the screw holes, and even more so in a dimly lit place.

Care must also be taken when removing the O-ring remaining on the carburetor side. Damaging the air mixture passage by trying to scratch it out with a precision screwdriver or similar tool can affect the flow rate of the air mixture, as can damaging the taper at the tip of the pilot screw. There is a pick-up tool that is ideal for this type of work, and should be utilized when O-rings are left in the body.

By paying attention to these points, the adjustment function of the pilot screw will be maximized and you will end up with a carburetor that is in good shape.