The carburetor setting is one of the highlights of tuning, as it allows you to change the air-fuel ratio by changing the jets and needles, along with replacing the exhaust system and chamber or customizing the engine itself. One of the most important factors in the carburetor setting is the "oil level in the float chamber". If it doesn't overflow, it's not ok, but it's important to understand why the height of the oil level affects the air-fuel ratio in the entire range.
Consider the height of the oil level to be the same as the height of the well
It is not uncommon for a carburetor that has been left in an inoperative state for a long period of time to need a full overhaul due to altered fuel and corrosion. Even if the jets and needles are not changed, check the oil level after disassembly.
In contrast to fuel injection, which includes parts that use electricity such as fuel pumps and injectors, the components of a carburetor are all analog. The carburetor, which suctioned out fuel according to the negative pressure generated by the engine, is simple and profound.
The role of the carburetor is to create a fuel mixture by suctioning fuel out of the jets and mixing it with air in response to the pressure and volume of air the engine draws in. Currently, fuel injection and does that job, but carburetors have been an integral part of the internal combustion engine since the birth of the internal combustion engine, which burns the mixture in the combustion chamber.
In contrast to fuel injection, which is a complex computer program and a solenoid valve that injects fuel pressurized by a fuel pump, fuel is interrupted by the float, metered by the small holes in the slow and main jets and the delicate taper angle of the jet needle, and The carburetor, which supplies the venturi, operates entirely on the negative pressure and flow generated by the incoming air.
Therefore, if the amount of air inhaled or exhaled changes significantly, such as removing the air cleaner box, replacing the exhaust system, enlarging the displacement by increasing the bore or replacing the camshaft, it is necessary to change the carburetor settings accordingly.
Depending on the throttle opening, the carburetors are roughly divided into the following areas: slow jetting from fully closed to one quarter, jetting needle from one quarter to three quarters, and main jetting from three quarters to fully open. Before the jets and needles, however, there is a fundamental adjustment factor in the carburetor setting. That is the float oil level.
Some riders may have experienced the panic of an overflow caused by debris in the fuel tank flowing into the carburetor and getting caught between the float valve and valve seat. Although you may only be aware of the fuel in the float chamber at such times, the oil level, which is maintained at a constant level by the float, has an important role to play.
The service manual specifies the height of the float oil level to a certain extent. An example that is easy to understand is shown by the distance from the mating surface of the carburetor and float chamber to the bottom of the float.
If the distance between the carburetor and the bottom of the float chamber is large, the oil level in the float chamber becomes low because the float floats and the valve closes when there is little fuel in the float chamber. On the other hand, if the distance from the carburetor to the bottom of the float chamber is close, the float floats and the valve closes when there is more fuel in the float chamber, so the oil level becomes higher.
Some users may consciously set the oil level lower as a safety measure, knowing from experience that higher oil levels are more likely to overflow. However, the oil level set by the manufacturer includes a crucial point of the carburetor setting apart from the presence of overflow.
I mentioned earlier that carburetors supply fuel in response to the negative pressure and air volume generated by the engine, and the ease with which fuel is released depends on the height of the oil level. Although we don't have the opportunity to see it around us anymore, if we replace it with a well that combines a pulley and a hand tub, a shallow well can be filled quickly, but if the well is deep to the water surface, it is difficult to lower and pull up the tub. The oil level in the float chamber is the same as in a well, and the height makes a difference in the ease of supplying fuel.
- Point 1: The oil level in the float chamber has a great influence on the setting as well as the setting parts such as jets and needles.
- Point 2: Depending on the height of the oil level, it is easy or difficult to produce fuel even if the same negative pressure is applied.
If the oil level is too high, not only will it overflow, but it will also affect the setting.
This float level gauge measures the distance from the bottom of the carburetor to the bottom of the float. A caliper depth gauge can be substituted, but two legs standing on the carburetor will stabilize the gauge when measuring.
Set the arm of the float level gauge to the number given in the service manual to measure the float height. Tilt the carburetor with the float pin pointing upward and measure at the point where the float lip touches the plunger at the rear of the float valve. On most carburetors, the main jet mounting point coincides with the bottom of the float.
If the arm of the gauge touches the float, the oil level is low; if the gauge swings out, the oil level is high. If adjustment is necessary, bend the float lip with a precision screwdriver. If the lip is bent diagonally, it will not be able to push the float valve evenly, so be careful not to twist it when adjusting.
If the well is shallow enough, so that the water can be pumped out easily. If the oil level is high, fuel is easily suctioned up even if the negative pressure is small. The opposite is true. On the contrary, if the oil level is low, the fuel can't be suctioned up unless the negative pressure is high.
If you do not change the diameter of the jet, there is no change in the 100% fuel flow rate that can pass through that jet. However, the amount of fuel that can be dispensed when negative pressure is applied from a zero flow state will increase or decrease depending on the height of the oil level. It is a racing carburetor, but in the Keihin FCR setting manual, it is stated that if the oil level is 2mm higher than the reference oil level, the volume increases by 7-8% at low speed, and if it is 2mm lower, the volume decreases by 7-8%.
The range of increase or decrease is supposed to converge above the medium speed range, but if the oil level is high, it will affect not only the slow speed but also the main system. However, if the oil level is too high, it will affect the main system as well as the throws. If you try to set the carburetor setting with the oil level out of the standard value, there is a possibility that it will not add up.
If you start setting the oil level higher than the standard value, the oil level will tend to be thicker in the pilot screw and air screw adjustment range, so close the pilot screw or open the air screw to thin it out. If the thick tendency continues in the slow jet range, you will have to lower the setting to make up the difference.
On the other hand, if the oil level is low, not enough fuel can be sucked up at idling, so open the pilot screw, close the air screw, and if it is still insufficient, increase the slow jet speed to increase the supply.
However, if the jets are tightened when the oil level is high, the absolute fuel flow rate will be reduced and the A/F ratio will be thin due to the lack of fuel at full throttle. Likewise, if the oil level is low and the jets are selected from low to high, extra fuel will be supplied at full throttle and the A/F ratio will become thicker.
Since the ironclad rule of carburetor setting is to start from the region where the throttle opening is small, the symptomatic treatment of narrowing the throttle if it is thick from the slow jet region is not wrong. However, as a result, if the main jet is thin in the full throttle range, it is difficult to know where to resolve the discrepancy. If you are only concerned about the number of jets and needles, and neglect the float height as long as it does not cause overflow, you may lose your way at the end of setting.
In order to avoid such pitfalls, it is important to always check the oil level when overhauling or setting up the carburetor. For singles, of course. The oil level must be aligned, even more so if you have a four-row carburetor. If the return speed of the pilot screw is different for one carburetor out of four carburetors, you should be careful enough to measure the float height again.
- Point 1: When the oil level is high, fuel is more likely to come out, so it becomes thicker, and when the oil level is low, it is harder to come out, so it becomes lighter.
- Point 2: It is important to align the oil level of each 4-carbons.
If it's a single-cylinder carburetor, you may be able to visually check the actual oil level.
If you prepare a glass container that can hold a float instead of a float chamber, you can check the rise of oil level and the working condition of the float. This method is possible because the oil level of most old-fashioned carburetors is lower than the mating surface of the carburetor body and float chamber. This is not possible with carburetors where the oil level is above the mating surface.
If the float valve is closed and the oil level is adjusted correctly, the fuel poured into the carburetor will float the float and the valve will close, and it will not overflow from the container. If the actual oil level is not correct even though it is adjusted to the standard value by the float level gauge, check that the float valve is not sealed properly or that the float is damaged. Since this float is made of brass, it is easy to find out if there is a hole in the float, but some floats made of foam resin lose buoyancy due to aging.
When the oil level in the float chamber is high, the distance between the float chamber and the carburetor passageway (bottom of the venturi) is closer, so it is easier for the float chamber to overflow than when the oil level is low. However, if the oil level is within the standard value set by the manufacturer, the overflow will not occur if the carburetor is tilted back and forth or left and right in normal riding. If the oil level is still overflowing, please check if the valve is tilted due to wear of the tip of the float valve or the float lip that the float valve contacts.
When the carburetor has been overhauled, it is also useful to check the actual oil level by pouring fuel into the carburetor before installing it in the engine to check for overflow.
If you connect a tube to the drain pipe and place it against the side of the carburetor body, the oil level in the float chamber and the oil level in the pipe will be at the same height, so you can see the actual oil level separately from the float height measured by the float level gauge. You can see the actual oil level.
For single cylinder carburetors, the float chamber can be replaced with a glass container to check the float operation and actual oil level. The actual oil level varies depending on the carburetor, but in most cases, it is below the surface where the carburetor body and float meet. When the float is placed on a container that does not interfere with the movement of the float and fuel flows through it, the container takes the place of the float chamber. When the oil level rises and the float floats, the float valve closes and the fuel stops.
If the oil level is below the carburetor body at this time, it is unlikely that overflow will occur when the carburetor is actually installed in the engine. On the other hand, if fuel overflows from the container even though the float is floating, it is necessary to recheck the cause of the overflow because it may be due to a poorly sealed float valve or insufficient float buoyancy.
However, some carburetors such as Kawasaki Zephyr 400 and Keihin CVK Zephyr χ have the specification that the oil level in the float chamber is above the mating surface of the carburetor and float.
When overhauling or setting a carburetor, it is important to understand that the float height is the most important and standard above all else, and in the case of two or more cylinders, it is important to match the oil level of each before setting the jets and needles.
If the overflow is found after the overhauled carburetor is installed on the engine, it will be troublesome to deal with, so fill the carburetor with fuel in a single state and check for leaks. It is important that the float valve is closed not only when it is placed on a stand but also when it is tilted back and forth or left and right.
On some carburetor with a drain pipe attached to the float chamber, the oil level in the chamber can be checked from the outside. In the case of an overflow pipe instead of a drain pipe, it cannot be measured.
The fuel level gauge has a glass tube with a scale engraved on the end of a rubber tube, and is connected to the drain pipe. The standard value varies depending on the type of carburetor, but it should be within some range above or below the matching surface.
- Point 1: In addition to the static level measured by the float level gauge, the actual oil level can be measured to grasp the oil level close to the reality.
- Point 2: If you have a container to replace the float chamber, you can check the movement of the float and float valve.
