Carburetors, which are familiar to owners of old and discontinued motorcycles, are simple yet profound, and their condition depends on the condition of the jets, needles, and other small parts. Particularly delicate are the starter and slow systems, which are used in areas where the throttle opening angle is small. Many owners think, when I say carburetor, I mean main jets and jet needles, right? The more you think that the carburetor is not the main jet and the jet needle, the more you may need to reconfirm the slow system.
If you are mainly riding around town, setting and tuning the slow system is more important than the main system
Comparison of carburetor before (left) and after (right) cleaning. Dirt adhering to the venturi may have reached the air ports at the bottom. Air sucked in from here flows into the slow jet and main jet sections mixes with gasoline metered by the jets, and is sucked out of the ports in a state where it is easily atomized. If the passage is narrowed by dirt in the air port, the air is reduced and the gasoline is increased relatively, so the mixture ratio tends to become thicker. However, if the jets are narrowed, the flow rate will not be balanced with the flow rate of the venturi, and the setting will be off.
Pilot screw with carbon buildup on the tip due to blowback. If the condition progresses further and carbon builds up, it may lead to a worsening of starting performance and reduced idling stability. When idling is not stable unless the return rpm is increased, remove the screw and check the condition. If carbon is deposited in this manner, it is recommended that carburetor cleaner be spread from the slow jet to the pilot outlet and slow port to clean them.
Along with the pilot screw, the starter plunger is important at low throttle opening. Basically, this part is used to create the thick air mixture required for cold starting, but as the sealing properties deteriorate over time, gasoline can flow even when the choke lever is pulled back, causing the mixture to be thicker at idle.
The rubber packing at the end of the starter plunger is the part that opens and closes the gasoline passage in the starter system, and if the choke lever is not pulled, the passage is closed for a longer time. During this time, the rubber may harden and become less airtight. In addition, in some long-term immobilized vehicles, the gasoline in the starter path may change and turn greenish-blue, as shown in the image, and opening and closing the starter in this state may damage the plunger or carburetor side, also causing a loss of airtightness.
The air mixture supplied by the carburetor varies with the amount of air intake and throttle opening, which increases or decreases with engine speed. Every rider knows that when the throttle is closed, the engine speed is low because the amount of intake air is low, and when the throttle is open, a large amount of air is sucked in, which increases the amount of air mixture and the RPM.
And when the throttle opening changes, the roles are autonomously performed inside the carburetor. When the choke lever is activated during cold starting, the choke mechanism, which forcibly shuts off the intake air, or the starter plunger, creates a thicker-than-normal air mixture to improve starting performance.
The slow system works mainly in the range from idle to around 1/4 throttle opening. The slow system uses a slow jet to meter gasoline and either an air screw or pilot screw to make adjustments. The pilot screw increases or decreases the mixture of air and gasoline in the carburetor.
When the throttle is opened more than 1/4 throttle, it is time for the main system to come into operation. Gasoline metered by the main jet is sucked out through the main nozzle protruding from the venturi section. The final amount of gasoline is determined by the gap between the main nozzle and the jet needle.
The interpretation of throttle opening is slightly different for piston valve type carbs, where the throttle wire opens and closes the throttle valve directly, and for negative pressure type carbs, where the wire operates the butterfly valve and the vacuum piston is activated by the negative pressure generated by the engine. This is because even if the throttle grip is marked so that the throttle opening can be read at hand, the butterfly valve opening does not always match the vacuum piston opening.
Despite these differences, what we want to focus on here again is the throttle opening used in everyday city riding. For a very short period of time when accelerating in a lower gear, the throttle opening may be wide, but when shifting up and getting into the flow of traffic, the throttle opening should be much smaller. On big bikes over 1000cc, it is not uncommon to find that the throttle opening is only about 1/2 open even though you think you are accelerating at full throttle.
This does not mean that the driver is lacking in spirit or determination, but rather that there are no situations in which the throttle can be maintained at full throttle as long as the car is driven in accordance with traffic rules. It is natural that a custom machine equipped with a high-performance carburetor would want to focus on the main system settings in order to bring out its full potential, but this does not mean that the slow system should be neglected. The same is true for negative pressure carburetors, which are often used in factory specifications.
Turning once again to the slow system, an important function is performed by the pilot screw (we will omit the air screw specification this time). With the throttle valve closed (open enough to idle), the mixture through the slow jet is carried to the pilot outlet, where the pilot screw finally adjusts the flow rate.
The service manual lists the standard pilot screw return speed and basically sets it to that number. In reality, however, the engine is started and the screw is fine-tuned to the highest engine speed. This is because, this is the optimum amount of gasoline for the amount of air intake at idling.
Next, when the throttle is opened from idle, the slow port opens along with the pilot outlet to supply gasoline metered by the slow jet. When the throttle is opened, a larger negative pressure is applied to the slow port than to the pilot outlet, thus reducing the influence of the pilot outlet.
The setting of a slow system that starts and accelerates from idle directly affects the ease of handling at low speeds. If the pilot screw has a low return rpm and the mixture ratio is thin at idle, there will be little power at the start of throttle opening and the engine will feel as if it has little power. On the other hand, if the return rpm is high, a thicker mixture flows when the throttle is opened, which tends to give the engine a gruff, jerky feeling.
In either case, the connection with the mixture flowing out of the slow port will be poor, causing the engine to feel unwieldy during start-up acceleration in areas where the throttle opening is small.
In the case of Keihin FCR carburetors, which have both a pilot screw and an air screw (slow air jet), the setting is difficult because the mixture from idling to opening is determined by three factors: slow jet, pilot screw, and air screw. In the case of a stock carburetor, the slow air jet is fixed, so if the slow jet is a stock standard size, the pilot screw alone is used for adjustment.
You may think that the mixture ratio will not change significantly then, but this is not always the case. The pilot screw is located on the engine side of the throttle valve and is subject to carbon buildup from the engine blowback. Since the pilot outlet is a very small hole and gasoline is sucked out through the narrow gap between the hole and the tapered end of the screw, carbon contamination can affect the flow rate.
When determining the pilot screw's return speed, tighten the screw until it stops and then returns it, but be careful not to tighten it too hard, as this may damage the tapered tip or the inside of the port. If the tip of the screw is damaged, the gasoline flow rate may be affected.
The pilot outlet and pilot screw affect the throttle from fully closed to about 1/4 throttle.
For bikes with large displacement, this is actually the area that is used the most.
It is important to suppress the desire to focus on the mains and reiterate the importance of the throttle system.
POINT
Point 1 - Parts inside the carburetor have different roles depending on the throttle opening.
Point 2 - The pilot screw and slow jet are responsible for a small throttle opening area, but their role is very important because they are the most frequently used parts of the throttle in city driving.
