Uneven Wear of Butterfly Valve Causes Idling & Low Opening Malfunction! Check the Gap by Facing the Detached Carburetor in the Sunlight

Even after repeatedly adjusting the pilot screw and tuning the throttle, the throttle still gets stuck at the start when opening the throttle from idle, or when closing the throttle after opening it. If you have such symptoms with a negative-pressure carburetor, it may be due to a worn throttle valve. Try facing the detached carburetor to the sunlight and see if the valve closes tightly and completely.

The throttle valve holds back the air that the engine is trying to suck in.

When the throttle drum is turned and the butterfly valve begins to open, check to see if the venturi and butterfly are rubbing against each other and making a strange noise, or if there is any part of the valve that has a metallic sheen due to interference or friction. When the throttle is opened just slightly, the same amount of air flows from all four carburetors, allowing the engine to blow smoothly. On the flip side, if the air is not evenly distributed at the start, the combustion conditions of the four cylinders will not be the same and smoothness will be compromised.

The most important part of carburetor maintenance and the setting is the adjustment of the slow system. In the days of fuel injection and mainstream carburetors in the racing world, there was a method of setting from the main system, but at leastSlow is paramount in city driving.

Furthermore, even if you are in the World GP class, you close the throttle when cornering (whether you close it all the way or not), and you use the slow system when exiting the corner. If the carburetor setting is not correct, such as a bumpy and jerky ride, or if the carburetor setting is not correct, then even if the maximum power is produced, it will not lead to a good lap time on the circuit, according to the words of a professional who did carburetor setting for the World GP.

Negative pressure carburetors were installed on many of the four-stroke models in the 250cc class and above manufactured since the 1980s. The operation of the throttle grip opens and closes the disk-shaped butterfly valve (throttle valve), and the negative pressure generated in the venturi, the air passage, causes the vacuum piston to move up and down, supplying gasoline. It is said to be superior in total performance compared to the piston valve type carburetor where the cable directly moves the piston up and down, which is used by minibikes, racing carburetors such as FCR and TMR, and out-of-print models such as CB750 Fords and Z1/Z2 generation.

Whether it's a negative pressure or piston valve system, when the throttle is fully closed the air flow is reduced to the minimum that will allow the engine to idle. Fully closed may seem like you are shutting off the air, but if you really shut off the air, the engine will stop. In the case of a 4 cylinder carburetor, the idle speed can be adjusted by turning the throttle stop screw, which is often located under the float chamber. By this operation, the opening degree of the butterfly valve when the throttle is fully closed is adjusted. When the screw is tightened, the throttle opens and the RPM increases, and when it is loosened, it closes and the RPM decreases.

Here's what you need to remember. The butterfly valve holds back the flow of air that the engine is trying to suck in. That's what I mean. When the butterfly valve opens, the engine speed increases, but the air is not sitting in the air cleaner box waiting to be sucked in. The strong negative pressure generated in the cylinder by the piston stroke is trying to suck in as much air as possible as soon as the valve opens.

It is similar to blocking the end of a vacuum cleaner hose with the palm of your hand and opening the gap little by little. If you close the hose completely, the bellows of the hose will shrink and the motor will growl in pain, but if you move your hand just a little, air will flow in through the gap, the shrunken hose will stretch, and the growl of the motor will become smaller. If you let go of the hose, the amount of air drawn in will be maximized and the negative pressure in the hose will be minimized. Roughly speaking, the relationship between the carburetor and the engine at idle is like this.

The throttle shaft and butterfly valve are non-disassembled and no genuine parts exist.

You can hold the carburetor up to the sun to check the contact between the venturi and butterfly valve, but a small light such as a clip light is also useful. The stronger the light, the easier it is to find even the smallest gaps.

The butterfly valve is opened a little bit larger to make it easier to see in the picture, but if you see light streaks like this leaking out when the valve is fully closed, you can tell that the butterfly is not sealed properly. The small hole at the bottom of the valve is a port for idling. Even when the butterfly is fully closed, the engine is still sucking in air through this hole.

The area at the base of the throttle shaft has a slower airflow rate than the upper and lower butterfly and is prone to carbon build-up. This carbon can act as a friction material and accelerate wear on both sides of the butterfly, so when cleaning the carburetor, use a foam-type carburetor cleaner to remove any dirt.

The butterfly valve is a hindrance to the engine that tries to suck in air at idling. Therefore, the role of the butterfly valve is very important, but it is a mechanical part, and wear by aging is unavoidable.

The throttle butterfly consists of a disk-shaped butterfly body and a throttle shaft that opens and closes the butterfly. in the case of a quadruple carburetor, the shafts of adjacent carburetors are linked and the opening and closing movements are synchronized and operated by a cable. The negative pressure type carb is designed so that when the butterfly is fully closed, the entire circumference of the butterfly is in close contact with the inner wall of the venturi.

The close contact of the butterfly with the inner wall of the venturi means that there is friction and wear here. It is impossible to drive without opening the throttle, so it is inevitable that the butterfly will wear, but if there is a gap, the air being sucked in cannot be managed and the engine may modulate. In a four-cylinder engine, the four cylinders will draw in different amounts of air and the carburetor may not be aligned with the pilot screw or tuning alone.

Checking the gap between the venturi and the butterfly is easy. Pull the vacuum piston out of the carburetor you removed from the engine, hold it up to the light and look through the venturi to see the amount of leakage from the outer edge of the butterfly. The area that tends to wear is at both ends where the throttle shaft goes through, and you may be able to see a rattle if you move it axially.

If the throttle is fully closed and the air is sucked in, even if the throttle stop screw is loosened to the limit, the idle rpm will not drop or the engine rpm will not drop when the throttle is returned. If you have a four-row carburetor, the pilot screw will have a different return speed, and even if the vacuum gauge is in sync, there may be a variation in engine speed when the throttle is opened from there. All of this is caused by air entering through the gap apart from the movement of the butterfly valve. As mentioned earlier, this is because the engine always wants to suck in as much air as possible.

Pilot screw adjustment and synchronization adjustment are the key to setting up idling and throttle low RPM range, but actually, the gap between butterfly valve and venturi is more important than that for negative pressure carburetor is the fundamental element. Unfortunately, the butterfly valve is a non-disassembled part, so it is difficult for the average user to get a replacement butterfly valve, except for carburetor overhaulers and some specialty shops.

Loosen the screw and adjust the position of butterfly valve and venturi.

The back of the screw has been punched to prevent loosening, and there is no need to remove the screw if you just want to improve the installation. You only need to loosen the screws about 1/4 to 1/2 turn to allow the butterfly to move freely. If the screw does not loosen at all due to the strength of the punch, use a rheometer or similar tool to scrape the tip of the open screw before working on it. Also, to prevent the unnecessary force from being applied to the shaft when pushing the screwdriver to turn, insert a stick from the other side of the venturi to hold the shaft in place.

After loosening the screws, open and close the valve with the throttle shaft to adjust the position of the valve against the venturi. If you change the angle of the valve too much, it will bite into the venturi, so be careful to adjust it slightly. After opening and closing the valve several times, the valve settles down to a comfortable position, and you can feel it hitting the venturi all the way around. Once the valve is in a good position and closed, hold it up to the light to check for leaks as you did the first time. This method doesn't work 100% of the time, but it often improves the hitting position and narrows the gap.

So the next best thing to do is....adjusting the position of the cylindrical venturi and the disk-shaped butterfly valve. This involves moving the butterfly that is screwed to the throttle shaft slightly so that it hits the venturi evenly. If a replacement butterfly is not available, positioning alone may not always give the best results, but it may improve the seating of both.

Although the screw that holds the butterfly valve is small in size, it must not be loosened during use, so it may have a punch on the backside to prevent it from turning. If you want to remove the valve, you need to remove the punch, but if you just want to loosen the valve to the extent that it moves, you may be able to do so without removing it. When loosening the screws, do not press too hard on the screwdriver as this will add extra force to the throttle shaft, so support the shaft with a round bar from the other side of the venturi while loosening the screws. The amount of loosening screws should be kept to the minimum to allow the butterfly to move. If the butterfly moves unsteadily, it will be difficult to find a good position.

After loosening the screws and allowing the butterfly to move a little more freely on the throttle shaft, lightly move the throttle shaft in the fully closed direction and push the butterfly against the venturi. The butterfly will then move on its own to a place where it is in good contact with the venturi. If you apply too much force in the closing direction, the butterfly will bite, but if you move the butterfly circumferentially while shifting the shaft left or right, the gap will be smaller.

Depending on the condition of the carburetor, this method may not improve the situation, but if you get a better feeling at idle and low rpm due to the butterfly valve settling at full throttle, your motorcycle will be easier to drive.

Some carburetor repair specialists manufacture their own butterfly valves and utilize them during overhaul. In this case, you can remove the valve from the throttle shaft. However, if the venturi side is worn out, all is not well.

Butterfly valves have different specifications for each carburetor, so they are not compatible with all negative pressure carburetors. The reason why we are able to manufacture these parts, which do not exist in the genuine parts, is because we have a somewhat limited number of models for which we receive many repair requests.