The amount of gasoline in the float chamber is kept constant by the float and the float valve. Therefore, when the carburetor overflows for some reason, the first thing to do is to check the condition of the float valve and valve seat, but if the float pin is handled incorrectly at this time, there is a risk of the carb itself becoming unusable.
Carb overflow is often caused by a malfunctioning float valve
The Suzuki RG250 Gamma was released during the racer replica boom era of the 1980s. Up to the 1987 model shown in the image, it was equipped with an aluminum double-cradle frame and a parallel two-cylinder engine, and in 1988 it was fully remodeled into the RGV250 Gamma with an aluminum twin-spar frame and a V-twin engine. The carburetor was equipped with a Mikuni VM28.
The layout of the cowl and aluminum frame is complicated and the removal of the carb is troublesome. The gasoline in the float chamber is deteriorated because it was left for a long time, but it is fortunate that it was kept in the warehouse and there is no corrosion.
Although all new motorcycles are fuel-injected, there are still many carbureted models in the world, from mopeds to big bikes. Whether you are running a scooter for shopping or transportation without being aware of the presence of a carburetor, or a special carburetor that maintains high performance while changing the setting every season, the most important thing is the gasoline oil level in the float chamber. Carb settings are made with jets and needles, but the oil level has a much more fundamental effect.
If the oil level is high and close to the bottom of the venturi, the gasoline in the float chamber is sucked out easily, and if the oil level is low, the gasoline is not sucked out easily. If the oil level of the carburetors for a four-cylinder engine is different, the amount of gasoline that is actually sucked out may be different even if the jet number is the same, resulting in a thicker or lighter mixture. For this reason, the service manual for each model lists checking and adjusting the oil level as the first step in carburetor maintenance.
The most troublesome trouble for such a carburetor is overflow.Overflow occurs when the oil level in the float chamber becomes too high, and the gasoline that exceeds the height of the overflow pipe in the float chamber is discharged to the outside of the carb.. It also has no overflow pipe to discharge to the outside.There are some types of carbs that allow gasoline that exceeds the specified oil level to flow into the venturiIn both cases, the float valve, which is opened and closed by a float that moves up and down depending on the oil level, fails to close properly. In both cases, the float valve, which is opened and closed by a float that moves up and down according to the oil level, fails to close properly, and gasoline flows endlessly from the fuel tank, resulting in an overflow condition.
If an overflow occurs, immediately shut off the gasoline flow and remove the carb to check the contact between the float valve and the valve seat, but this operation requires removing the float pin that supports the float.There are several types of float pin assembly methods such as simple through type, press-fit type and flanged screw type.
- Point 1・Carburetor overflow is often caused by poor sealing of the float valve.
- Point 2: Depending on the structure of the carburetor, the overflowed gasoline may flow out of the carburetor or overflow into the venturi.
If the pin is press-fit, carefully tap it with a pin punch
The float pin is lightly press-fitted with a flange on one side like the head of a nail. It is easy to see the direction of attachment and removal because of the flange, but if you try to pull the pin forcibly with pliers, it may damage the pin or the pin foot. The same direction should be used when inserting the pin. When the base of the pin on the flange side is tightly pressed in, tap the pin with the socket supporting the bottom of the foot so that downward force is not applied directly to the foot.
If the float pin is pulled out, the float and the float valve can be removed. When overflow occurs, the first thing to do is to check the condition of the contact surface between the float valve and the valve seat, but if the valve seat is removable as shown in the picture, check if the O-ring between the valve seat and the carb body is thin or broken.
The float pin, which is the axis on which the float pivots, must not come loose by mistake in the float chamber. In this respect, it makes sense to press-fit the pin into the carb body. However, pulling out a press-fit pin requires a level of caution incomparable to that of a non-press-fit type pin. There are several patterns of troubles at the time of carburetor overhaul, such as the female screw that fixes the float chamber is nappy, or the screw head of the jet is crushed and cannot be loosened, but there is no trouble as critical as the leg of the float pin breaks. If you apply force to a press-fit pin with the same intention as a non-press-fit type pin, it may break surprisingly easily.
It is easy to determine whether the float pin is a press-fit type or not. If it is a non-press-fit type, it should come out easily by pushing it with a precision screwdriver or pin punch. It may even come out by simply tilting the carb body. On the other hand, the press-fit type pin will not budge when pushed with a screwdriver, and must be pushed out with a pin punch of the same thickness as the pin. In the case of press-fit type pins, most of the pins are press-fitted into only one of the legs where the pin is inserted, so it is important to carefully check the fit of the leg and pin and determine the direction in which to strike the pin with the pin punch. In some cases, one side of the pin is formed flat like the head of a nail, so you can tell at a glance which direction to push it out.
When tapping out a press-fit float pin with a hammer and pin punch, it is important to firmly support the foot of the tapping side and the opposite sideIn this case In this case, by placing the opening of a smaller socket such as 5mm or 8mm at the center of the pin on the pull-out side, the pin can be pulled out without being disturbed while receiving the impact when the hammer strikes the pin. Also, the hammer that strikes the pin punch should not be swung down, but should be so careful that it is weighted only by the hammer's own weight.
If you have a twin carb with a body that can be easily separated as shown in the picture, it is easy to support the feet of the float pin, but if you have a four-cylinder carb, it may be difficult to set the socket to support the feet. In such a case, even if you feel it is troublesome, you need to be careful to split the four bodies for each cylinder and pull out the pin after making the situation to support the foot stably. Even if the press fit is tight, if you hit the pin hard in an unstable condition and hit it in the wrong place, the pin leg will break more easily than you think.
However, if only one leg is broken, it is difficult to align the height of the pin hole with the remaining leg, and if both legs are broken, it will affect the movement of the float, making the repair even more difficult. Nevertheless, if you need to check the float valve, one way is to ask a reliable professional.
- Point 1・When the float pin is pressed in, do not try to pull it out forcibly, but tap it out carefully with a pin punch.
- Point 2 - If the carb for a 4-cylinder carb is a press-fit pin, disassemble the connection to make individual bodies and then remove the pin.
Cracks in the overflow pipe can cause overflow!?
The pipe that is press-fitted into the bottom of the float chamber on the right side of the image is the overflow pipe, and when it cracks longitudinally due to aging, the gasoline in the chamber leaks out and becomes an overflow condition. If this is overlooked and mistakenly thought to be caused by the float valve, the symptoms will not improve even if the carb is restored by sliding the float valve and valve seat.
As explained earlier, the cause of overflow is gasoline flowing in due to a poorly sealed float valve, but overflow can also occur due to causes other than float valve failure. A typical example of this is the failure of the overflow pipe in the float chamber. When the oil level rises too high due to a poorly sealed float valve, gasoline flows into the pipe and is discharged from the drain pipe in the float chamber. It is a pattern.
The material of this overflow pipe is often a brass-based alloy, but it may be made of a plate that is rounded and joined at both ends.If this joint is cracked due to age-related deterioration, the oil level will leak from the drain pipe even if it does not reach the top of the overflow pipe.It is easy to overlook a hair-thin crack. Hair-thin cracks are often overlooked, and when an overflow occurs, the float valve is not noticed and only the float valve is inspected and replaced, and when the carb is restored, it leaks again ......, especially in older out-of-print cars.
In such a trouble, it is common to seal the cracks with solder or epoxy resin. If water has entered the float chamber and the overflow pipe is perforated by corrosion, the pipe itself may be replaced with another brass pipe. In this case, it is important that the height of the pipe matches the dimensions before replacement.
Although there is no significant difference in the structure of the float or float pin between mopeds and big bikes, it is important to carefully observe the float chamber once it is removed, as the difficulty and preparedness of the work will depend on whether the pin is press-fit or not.
- Point 1・Even if the float valve is normal, it may overflow due to damage to the overflow pipe.
- Point 2 - Repair the cracked overflow pipe with solder or epoxy resin.
