The carburetor still plays a leading role in the intake system of an out-of-print car. It is often thought that most of the malfunctions are concentrated in the fuel system, such as clogged jets and poorly sealed float valves, but as the mileage increases, wear in each part also progresses. Let's take a look at the causes of carburetor malfunctions through examples of worn air system parts.
Worn throttle valve can cause sluggish idling
The downdraft carb, which runs in a straight line from the air cleaner to the combustion chamber, is a symbolic part of a high performance engine. The float chamber and float valve are designed to keep the oil level stable when the carb is tilted forward, but it is important for out-of-print car owners to know that unexpected parts can wear and deteriorate over time.
The butterfly valve is only slightly open at idle because the engine would stop if the venturi was completely blocked and air was shut off. The up and down gap of the venturi is necessary, but the left and right gap where the throttle shaft goes through is a problem. When the throttle is fully closed, the butterfly and venturi are in close contact and rub against each other as the throttle is opened and closed. When the throttle shaft is closed, the butterfly and venturi are in contact with each other in the left and right directions.
If we look at the way you adjust the amount of air that passes through the venturi of your carburetor.Can be classified into throttle valve type and negative pressure type (CV type). In the throttle valve type, the throttle valve of cylindrical or square cross section is opened and closed directly by the throttle operation, and in the negative pressure type, the butterfly valve of disk shape which obstructs the venturi diagonally is opened and closed by the throttle operation, and the vacuum piston rises by the intake negative pressure which changes by it, and air and Gasoline and air are sucked into the engine. The throttle-valve type is used on small-displacement mopeds and 2-strokes, while the negative pressure type carb is used on most 4-strokes since the 1970s.
The negative pressure type is considered to be easier to handle than the throttle valve type because the vacuum piston and butterfly valve operate in two stages. Even if the throttle is opened suddenly and widely, the vacuum piston does not rise until the negative pressure generated by the engine is high, and when the engine rotation is increased by the gasoline sucked out from the slow port under the throttle valve, the vacuum piston finally rises, the jet needle is exposed, and the gasoline is sucked out from the main nozzle. Gasoline is sucked out of the main nozzle. On the other hand, in the throttle valve type, the throttle directly opens and closes the piston that determines the degree of opening of the venturi, so if the throttle is opened wide at low engine speed, not enough gasoline is sucked out of the main nozzle because the negative pressure of the engine has not increased, and the mixture becomes thin and the engine stalls.
For these reasons, negative pressure carbs have become the mainstream for production vehicles, butThe throttle valve itself may wear out after prolonged useThe engine is always trying to suck in as much air as it can while starting. Since the engine is always trying to suck in as much air as it can while it is running, the negative pressure on the carb is greatest at idle when the throttle is closed. The negative pressure carb, which has a disk-shaped valve blocking the venturi, is also trying to suck air through a small gap to increase engine speed. And.The throttle valve opens and closes in contact with the inside of the venturi, and as it wears slightly and becomes less airtight when fully closed, air is sucked in despite the throttle being fully closed, and the engine speed tries to increase.I'm not sure if this is a problem or not, but I've seen it happen before. Also, when the throttle is pulled back while driving, the air in the venturi is not completely shut off, resulting in a slow engine speed drop.
Since the throttle valve, throttle shaft and venturi are constantly exposed to air and gasoline and cannot be lubricated with oil or grease, structural wear is inevitable. When the idling speed cannot be fully adjusted by the throttle stop screw, or when the intake negative pressure becomes difficult to stabilize when measured with a vacuum gauge, check the throttle valve for wear.
- Point 1 - There are two types of carburetors for old and out-of-print cars: piston valve type and negative pressure type.
- Point 2 - If the diaphragm of a negative pressure carb is worn out and there is a gap between the carb and the venturi even when the throttle is fully closed, the engine rpm will not drop and the rpm drop will be poor.
The diaphragm of the vacuum piston can crack.
By using a flat valve vacuum piston in a negative pressure carburetor, the front and rear length of the carb can be shortened, leading to improved throttle response. It also has an advantage over cylindrical pistons for larger diameter venturi. The reason why there are so many rub marks on the lower part of the valve is because the pressure in the venturi fluctuates a lot when the vacuum piston opening is small, causing the piston itself to rock back and forth.
The two vacuum pistons are different from each other because of different diaphragm. The right one is more flexible and responds better to the change of intake negative pressure. However, negative pressure from air cleaner box and return spring setting will change the way of piston movement.
The vacuum piston, which is the reason for the negative pressure carb, is also an important point to check due to aging. When the throttle valve opens and the negative pressure of the engine becomes largeThe air in the throttle valve is sucked out through the suction port (hole at the bottom of the throttle valve). The difference in atmospheric pressure causes the piston to rise and more air to flow through the venturi.. When the throttle valve closes and the air flowing through the venturi is reduced, it is closed by the tension of the spring inside the piston.
The airtightness of the vacuum piston is maintained by a thin rubber disk-shaped membrane called a diaphragm. In the past, CB450 and CB750/900/Although there are some examples of metal pistons used in some Honda cars such as the 1100F, the majority use rubber diaphragms. Each carburetor manufacturer selects a material that maintains flexibility over a long period of time, but because it is a rubber material, it can harden over time and crack where it is bent. It is unclear whether or not being used for 20 or 30 years after manufacture was included in the design conditions, but given the current popularity of out-of-print bikes, it is not surprising that bikes of that age are still in use. After all, even the 2000 model is a full 20 years old.
If the diaphragm is torn, the air in the throttle valve is sucked out through the suction port, but the air in the venturi is sucked in through the crack, so there is no negative pressure in the valve and the piston will not rise!If the diaphragm hardens, it will resist the piston rise. Also, when the diaphragm hardens, it becomes a resistance to the piston rise and the piston movement becomes poor. Even if the throttle valve is opened, if the vacuum piston does not rise, the amount of air inhaled will not increase, so the engine speed will not rise. The trouble is, when only one diaphragm of a 4 cylinder carb is broken, the piston is fully closed when idling, so the engine is stable, but when it starts running, there is no feeling of punch, or the blow-off is uneven. If this happens, remove the top cover of the carburetor, take out the piston, spread the diaphragm and check for cracks.
If there is any damage, it needs to be replaced, but since the diaphragm and piston are integrated, they should be replaced at the same time. For older or out-of-print models for which the genuine parts are no longer available, there are external manufacturers that sell only the diaphragm as a single item, so you may want to search for them. Also, some users of models for which new parts are not available can reuse the diaphragm by gluing a thin rubber sheet to the crack. Because of the change in the characteristics of the diaphragm, the performance may not always be the same as a new part, but if you think it is better than being immobilized, it may be an option.
- Point 1: If the diaphragm of the vacuum piston is hardened or cracked, the piston does not move well and the engine speed does not increase even if the intake negative pressure is applied.
- Point 2 - In principle, a defective diaphragm should be replaced with a new one.
Jet needle wear thins the setting.!?
Compared to the jet needle on the left, the one on the right is worn to the point where the bottom of the collar is gouged. Because of this, the rattle when inserted into the vacuum piston is excessive and may hit the inner wall of the needle jet. The gasoline in the float chamber is metered by the main jet and is sucked up through the needle holder and into the venturi through the jet needle, but if the needle is still against the jet, not enough gasoline will be sucked up and the gasoline will be too thin for the air. However, if the needle remains against the jet, not enough gasoline is sucked up, and the gasoline becomes thin against the air.
Along with the butterfly valve, the jet needle, which is not thought to wear out, can also affect the condition of the carb through wear. The jet needle, which is inserted into a hole in the bottom of the vacuum piston, is not completely fixed but has a certain amount of rattle and that freedom allows it to sit in the center of the needle jet.
The pressure in the carburetor is constantly fluctuating, and the needle jet is also greatly affected by the air passing through the venturi, and will try to push against the wall behind the jet needle as the intake air volume increases. Some models incorporate a small spring between the top of the needle and the needle holder, and that spring is responsible for pushing the needle back to the center as it tries to tilt.
However, such a function may not be able to overcome the passage of mileage and time, and the contact part of the vacuum piston and the jet needle may become worn and thin. Then, the inclination of the needle due to the negative inlet pressure becomes more pronounced and it becomes more difficult to return to the center from the inclined state. When the vacuum piston rises due to negative inlet pressure, the jet needle tends to fall into the wall behind the needle jet, making it difficult for the gasoline sucked up from the needle jet to exit the venturi, and the carb setting may become thin. It is often thought that the same amount of gasoline will be sucked out if the jet opening area is the same even if the jet needle sticks to the needle jet, but it is difficult for gasoline to come out because the front of the jet needle is subjected to positive inlet air pressure.
Even if you are not aware of any specific malfunction, if the base of the jet needle is worn and stepped, the needle may be prone to pivoting a lot. If this is the case, it is necessary to replace the needle as soon as possible after finding it, because it is more likely that the needle keeps hitting the jet and not enough gasoline is being sucked out as the vacuum piston is opened more.
- Point 1 - The jet needle is positioned at the center of the needle jet by moving freely to some extent.
- Point 2: If the contact area with the vacuum piston becomes worn and thin, the needle will move more, sticking to the needle jet and making it harder to get gasoline out.