It is true that setting up a carburetor requires a certain amount of experience to do it well since the mixture ratio is judged darker or lighter based on plug burnout and engine feel. On the other hand, overhauling, which involves disassembly and cleaning, can be practiced by many riders with interest and concern. Here we describe some of the mechanisms encountered during carburetor overhaul.
What is the doughnut-shaped part inserted in the bottom of the float chamber for?
Carburetors for single-cylinder engines are often easier to install and remove than four-cylinder carburetors for four-cylinder engines. Even so, older models with a compact overall body design often require advanced preparations such as shifting the air cleaner case backward and removing the clamp band.
The factory carburetors for production vehicles do not have outstanding performance aspects, but they are built to perform well in any environment or used condition. While aftermarket high-performance carburetors are attractive, it is important to maintain the OEM carburetor to bring out 100% of its performance.
The pilot outlet, which opens at the boundary between the fully closed throttle valve and the bottom of the venturi, is where the mixture adjusted by the pilot screw is sucked out and is under the strongest negative pressure when the engine is started. As the throttle valve opening becomes larger, the negative pressure becomes lower, and the amount of air discharged from the pilot outlet decreases.
The pilot outlet, where the mixture is sucked out when idling with the throttle fully closed, is subject to blowback carbon as can be seen in the image above. The carbon also enters the inside of the outlet and adheres to the tip of the pilot screw. If the carbon makes the tip of the needle thicker, the air-fuel mixture passage becomes narrower, which can result in a thinner air-fuel ratio and poor starting performance. Therefore, the inside of the screw and pilot outlet contaminated with carbon must be cleaned with a carburetor cleaner.
The plate installed in the float chamber makes it difficult for gasoline to escape from the lower part of the main jet even if the vehicle body is tilted significantly back and forth, left and right. As long as the oil level is maintained, gasoline will continue to be supplied through the cross slits in the upper part of the plate. The removable drain bolt itself acts as a gasoline reservoir, but the plate prevents gasoline from running out of gas while keeping the overall height of the carburetor down by damming up the gasoline around the jet.
The plate is shaped to prevent gasoline from escaping around the main jet. It is fine to remove the plate because it gets in the way of cleaning debris from the float chamber, but it must be installed after cleaning. For riders who do not do wheelie or action riding, it may seem useless, but it is included in the carburetor assembly and does not exist on the parts list.
The role of the carburetor is to create a gasoline mixture by sucking up the gasoline in the float chamber through the negative pressure and flow of the intake air passing through the venturi. As explained many times, it is important to stabilize the oil level in the float for this purpose, but while the bike is running, it tilts back and forth, left and right, and the carburetor tilts accordingly.
The banked attitude of on-road models also affects the oil level considerably, but off-road models and trial vehicles that wheelie to near vertical are even more severe. If the gasoline in the float chamber moves from the bottom of the float and the jet is separated from the gasoline, the model will run out of gas.
To prevent running out of gas even if the vehicle changes its posture, the jet suction part is located near the bottom of the float chamber, far from the bottom of the venturi. Some carburetors, such as Keihin FCR carburetors and Mikuni TMR carburetors, have a cup-shaped deep drain bolt.
While there is the advantage of being able to change jets without removing the float chamber when setting the carburetor, there is also the purpose of using the drain bolt as a gasoline reservoir. If the bottom of the float chamber is flat, gasoline will flow at once when the angle of the carburetor (carburetor) changes due to banking or wheelie during cornering.
However, with a cup-shaped drain bolt, gasoline in the float chamber can flow in one direction but remain in the cup, thus avoiding out-of-gas conditions for a certain period of time. The greater the change in the attitude of the bike, the more effective the gasoline reservoir will be.
Similar to the removable drain bolt, a component called a plate (in the case of Yamaha) is installed to stabilize the oil level in the float chamber. It may be attached to the float chamber or it may be attached to extend the jet in the carburetor. In both cases, it is installed to mitigate the tilt of the oil level when the vehicle body posture changes significantly and to allow the jet to continue sucking gasoline.
At first glance, it is difficult to understand the purpose of this part, but it is a part installed by the carburetor manufacturer or motorcycle manufacturer according to the characteristics of the model, so do not remove it without permission. Although it may seem to interfere with the flow of gasoline since it is located near the main jet, this part cannot possibly reduce the flow rate.
POINT
Point 1 - A gasoline reservoir is provided at the bottom of the jet so that gasoline can continue to be sucked from the jet even if the bike's posture changes significantly.
Point 2 - Float chamber bottom and carburetor plates are installed to control oil level movement in the float chamber
Some carburetors have a filter attached to the float valve seat
There are two types of float valve seats that function as a set with the float valve: those that can be removed from the carburetor and those that are press-fitted and cannot be removed. The non-removable type does not necessarily have a valve seat filter. Even if the valve seat is removable, some types come with a filter and some do not.
The gasoline tank is not rusted and rust has not fallen off, but there is thread-like debris that looks like hairballs. If this debris gets caught between the float valve and the valve seat through the valve seat, the oil level may continue to rise with a slight gap even after the float rises and the valve closes, and if the oil level is higher than the overflow pipe in the float chamber, it will leak to the outside of the carburetor. The O-ring at the base of the filter is also badly cracked around the periphery when gasoline is wiped off, and if reused, gasoline may continue to flow into the float chamber through the gap between the valve seat periphery and the carburetor, so the O-ring replacement is essential
Over time, rust forms in the gasoline tank and flows into the carburetor, causing overflow if it gets trapped between the float valve and valve seat. Rust and small debris accumulated in the float chamber can also clog the jets, causing engine malfunction.
It is not uncommon for old or discontinued motorcycles to be able to start when the choke is used, but when the motorcycle is sufficiently warmed up and the choke is turned back on, the motorcycle will not idle and will stall.
In such cases, it is common to add a filter in the middle of the fuel hose, but some carburetors have a filter like a miniature tea strainer attached to the valve seat. In this case, the filter must be attached upstream from the float valve (on the gasoline tank side) to serve its purpose, so the valve seat is a removable type and the filter fits inside the carburetor.
Therefore, many users are unaware of the existence of the filter, but when rust that has flaked off from inside the tank passes through the fuel cock and clogs this filter, the rate at which gasoline accumulates in the float chamber is reduced.
To prevent such problems, a filter is also installed at the entrance of the fuel cock pipe inserted inside the tank. However, if the filter is torn over time or removed when the cock is attached or detached, rust and debris will go directly to the carburetor.
If you have no problems while idling or running at low RPMs, but you experience out-of-gas symptoms when running at high RPMs, it is tempting to suspect the carburetor setup and change the jets or needles. However, it is not uncommon for the cause of the problem to be insufficient gasoline flow or a clogged valve seat filter.
Whether or not a valve seat has a filter can be determined first and foremost by whether or not the valve seat can be removed from the carburetor. A carburetor with a press-fit valve seat does not have a filter itself, as the filter cannot be maintained. On the other hand, even if the valve seat is removable, it does not mean that 100% of the filter is attached. If the valve seat can be removed, actually remove it to check for the presence of a filter, and if rust or debris is present, clean it with parts cleaner.
In addition, the condition of the O-rings should be checked when the valve seats are removed.
Rubber parts on out-of-print vehicles often deteriorate over time, and the O-rings that fill the gap between the valve seat and the body also tend to deteriorate. O-rings that have remained in place for many years without being removed have become bent and retain their sealing properties, but once they are removed and parts cleaner is applied, the deterioration is often exposed and they cannot be reused.
Once removed, it is preferable to replace the O-ring with a new one without reusing it, but note that O-rings are not always sold individually for some models.
The carburetor shown in the image is for a Yamaha Tricker, but this model has a valve seat float valve, filter, and O-ring as a set of parts, and it is not possible to obtain just the O-ring. In this case, the standard practice is to purchase the valve seat assembly, but you can also use the Key Star fuel adjustment kit.
The fuel adjustment kit includes rubber parts such as O-rings for pilot screws, float chamber gaskets, and starter plungers as well as for valve seats. Considering that the kit includes jets and needles for the carburetor setting, it has the advantage of being more reasonably priced than the OEM parts.
While the availability of O-rings requires some thought, it is important to know that some carburetors have filters attached to the valve seats, which is important from both a maintenance and setting perspective.
POINT
Point 1 - Some carburetors with valve seats that are removable from the body have a filter at the valve seat entrance
Point 2 - When valve seats are removed from the carburetor, replace the O-rings around the valve seat circumference