Ethan Clarke
Adjusting the float height on a carburetor can help fix issues with your bike's fuel supply, which can cause the bike to run rich or lean. The float height is the distance between the top of the float and the closest point to the float when the needle valve assembly closes the fuel supply. To adjust plastic floats, you may need to heat up the plastic and press an indent where the needle sits, or use adjustable floats with metal hinges. There are also methods to determine the fuel level and adjust the float height by using an adapter and clear fuel line attached to the carburetor's drain hole.
How to adjust a plastic carburetor float
| Characteristics | Values |
|---|---|
| Method to determine fuel level | Use an adapter that fits into the drain hole of the float chamber |
| Adapter attachment | A clear fuel line with graduations is fitted to the adapter |
| Action after turning on the fuel | Wait for the fuel to flow up the pipe until the needle valve assembly closes the supply |
| Float height adjustment | Heat up the plastic and press an indent into the top where the needle sits |
| Distance between the top and the closest point to the float | 1/8" for one float, 7/32" for the other |
| Gas level | 0.40 below the bridge on one and 0.525 in the rear |
| Float level lowering | Add thin metal shim washers under the needle's seat |
| Float level raising | Add gaskets between the float bowls and their lids |
| Float height measurement | Measure the height when the float just closes the needle valve |
| Carburetor position | Carburetor should be turned right side up |
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What You'll Learn
Heating the plastic and pressing an indent where the needle sits
To adjust a plastic carburetor float, one method is to heat the plastic and press an indent where the needle sits. This method is useful when the plastic floats do not have a metal tab to bend. The first step is to apply heat to the plastic to make it pliable. This can be done by using a heating device such as a screwdriver, chisel, or nail stuck into a hot flame. Once the plastic is softened, it can be moved and held in its new place. It is important to be careful when applying heat to the plastic, as it can be easy to break the tab. After the plastic has cooled, it will likely stay in its new position.
It is worth noting that adjusting the plastic carburetor float in this manner is not a 'factory-approved' solution. In most cases, the floats are so precise that they do not require any adjustment. If the plastic float is off, it may be due to worn parts such as the needle, seat, float pivot pin, or the 'hinge' section of the float. Additionally, if water got into the gas and froze during winter, it could have modified the float setting or cracked the plastic.
Before attempting to adjust the plastic carburetor float by heating and pressing an indent, it is recommended to check for any worn or damaged parts that may be causing the issue. It is also important to be cautious when applying heat to the plastic to avoid breaking the tab or damaging other components.
Overall, heating and pressing an indent on the plastic carburetor float can be a potential solution to adjust the float height. However, it is important to approach this method with caution and to first rule out any other potential causes for the issue.
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Using shims and gaskets to adjust float levels
To adjust the float levels in a carburetor, you can use shims and gaskets. This method is particularly useful when dealing with non-adjustable plastic floats that lack a metal tab to bend.
When looking to lower the float level, thin metal shim washers can be added under the needle's seat. This adjustment helps achieve the desired specifications. Conversely, to raise the float level, gaskets can be inserted between the float bowls and their lids.
It is important to note that even a slight change in thickness can significantly impact the float level due to the geometry of the carburetor. As such, experimentation may be necessary to achieve the desired outcome.
For example, in the case of HS4 plastic floats, the fuel level should be at the top of the jet, approximately 0.070" down. The height below the bridge is recommended to be 0.16" +/- 0.04", with a low setting of 0.20".
Additionally, when measuring the float height, it is crucial to tilt the carburetor until the spring on the float needle begins to compress. This "sweet spot" ensures accurate measurements and prevents issues with fuel levels.
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Determining the fuel level with an adapter and clear fuel line
The fuel level can be determined by using an adapter and a clear fuel line with graduations. This method is called the external measuring pipe method. Here are the steps to follow:
Firstly, it is important to understand how the carburettor works. The float chamber acts as a reservoir for fuel, with the floats "floating" on the fuel. The floats pivot on a rod and open or close the needle valve, controlling the fuel entering the chamber. When fuel is drawn up, the float and fuel level drop, opening the needle valve and allowing more fuel to enter. As the fuel level rises, the needle valve closes off the fuel supply.
Now, to determine the fuel level, an adapter is fitted into the drain hole of the float chamber. A clear fuel line with graduations is attached to this adapter. The graduations on the fuel line allow for easy measurement of the fuel height. Once the fuel is turned on, it will flow up the pipe until the needle valve assembly closes the supply. The fuel height in the tube will then correspond to the fuel height inside the float chamber.
It is important to note that some carburettors have a drilled protrusion on the base of the float chamber where the clear fuel pipe can be attached, eliminating the need for an adapter.
Before conducting this test, it is recommended to check the float level, as it greatly affects the fuel mixture. The test involves connecting the clear fuel line to the carburettor drain valve, opening the drain valve, and turning on the fuel supply. The fuel level in the clear tubing should reach a certain height, indicating the float height before shutting off the fuel flow into the bowl.
It is worth mentioning that a float level slightly higher than the specification can negatively impact idling and cause issues when decelerating quickly. On the other hand, a slightly lower float level does not seem to affect performance noticeably.
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Blowing into the fuel supply pipe while lifting the floats
To adjust the float height of a carburetor, you must first remove the float chamber bowl from the main carburetor body. With the chamber removed, tip the carburettor onto its side, in the same position it takes while fitted to the bike.
Now blow into the fuel supply pipe while slowly lifting the floats until the airflow stops. This is the measuring point for the specified float height. The float height is the distance from the float chamber face or gasket surface to the highest point of the floats. This distance can be found in the manufacturer's manual, or in shop manuals.
If the floats are not at the right height, the tang on the floats should be bent towards the needle valve to allow less fuel in (making the bike run leaner) or away from the valve to increase the fuel level (making the bike run richer).
The float is a simple device that controls the amount of fuel that enters the float chamber, restricting or cutting off the supply by its settings. The float chamber is a reservoir for fuel and contains all the working components of the float.
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Adjusting the carburetor angle to measure float height
To adjust the carburettor angle to measure float height, the first step is to determine the current settings. This can be done by using an external measuring pipe or by measuring the physical height of the float above the gasket face. This second method involves using an adapter that fits into the drain hole of the float chamber. A clear fuel line with graduations is attached to the adapter, and once the fuel is turned on, it will flow up the pipe until the needle valve assembly closes the supply. The fuel height in the tube will correspond to the fuel height inside the float chamber.
Now, to adjust the carburettor angle, tilt the top of the carb to about 10 or 2 o'clock. This will allow the floats to fall back enough to close the float valve without squeezing the float valve spring. It is important to not compress the floats any further. The carburettor should be angled rather than straight up and down for an accurate float measurement.
Once the carburettor is angled correctly, measure the float from the base of the exposed lip (or carb body) of the carb to the top of the float.
If the float does not measure to the manufacturer's specifications, you can adjust it by bending the tang up or down to get the desired float height. The float height determines the amount of fuel allowed into the engine, so a correct float height is important to ensure the bike runs on the exact amount of fuel it needs.
If you are dealing with non-adjustable floats, you can lower the float level by adding thin metal shim washers under the needle's seat. To raise the float level, add gaskets between the float bowls and their lids.
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Frequently asked questions
You can adjust the float height to fix your bike from running rich or lean. This can be done by heating up the plastic and pressing an indent into the top where the needle sits.
The float height should be measured when the float just closes the needle valve. The top of the jets should be 0.060-0.080 below the bridge.
You can use an adapter that fits into the drain hole of the float chamber with a clear fuel line attached. With the chamber removed, tilt the carb onto its side and blow into the fuel supply pipe while lifting the floats until the airflow stops.
You can add thin metal shim washers under the needle's seat to lower the float level. To raise the float level, add gaskets between the float bowls and their lids.
A rich or lean condition can cause the engine to run poorly, with symptoms such as a rough idle, hesitation upon acceleration, and black smoke from the exhaust.
