Maya Ellison
Honda's CB750K model, released in 1979, has been a popular motorcycle for decades, but some owners have experienced issues with its float height settings. The manual states the height should be 15.5mm, but it is unclear whether this is measured with the carbs lying flat or on their side. Some owners have reported that their floats are not engaging the needle, causing fuel to overflow. This has been attributed to faulty needles or seats, with some opting to replace them with cheap plastic floats, which do not require adjustment.
| Characteristics | Values |
|---|---|
| Float height | 15.5 mm |
| Carburettor fuel measurement | The Sea Raven |
| Carburettor position | Upside down |
| Float bowl position | Facing up |
| Float material | Plastic |
| Carburettor type | CV type |
| Carburettor year | 1977 and 1978 |
| Gauge thickness | 5.3 mm |
| Gauge setting | 26 mm |
| Valve section position | Side or upside down |
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What You'll Learn
Check for float height issues
Checking for float height issues on a CB750K involves a few key steps. Firstly, it is important to understand the correct float height measurement. The standard height for the float is typically given as 15.5 mm, although some sources suggest it could be 12.5 mm or 22.5 mm. This discrepancy may be due to the orientation of the carbs during measurement, as some individuals recommend having the carbs upside down, with the float bowl facing up, while others suggest placing the carbs on their side.
Once the correct height is established, the next step is to remove the float bowl and set the carbs in the preferred orientation. It is crucial that the float rests on the spring-loaded fuel needle at the specified height. This process may be easier with the carbs upside down, as some have reported challenges in achieving the correct height with the carbs on their side.
The CB750K's plastic floats are designed without any obvious height adjustments, which is different from the metal tangs on older float designs that could be bent to adjust the height. This newer design indicates that adjustment may not be necessary, and the floats will maintain their correct height without intervention.
To verify the float height, one can utilise a simple template made from the mounting plate of the float bowl. By placing this template over the carb setup, it is possible to ensure that the floats are correctly positioned relative to the fuel needle. This process requires careful attention due to the spring-loaded nature of the needle.
In summary, checking for float height issues on a CB750K involves understanding the correct float height, removing the float bowl, positioning the carbs correctly, ensuring the float rests on the fuel needle at the specified height, and verifying the setup with a template. By following these steps, one can ensure the float height is accurate and address any potential issues.
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Inspect float bowl and needle
Inspecting the float bowl and needle on a CB750K motorcycle is an important step in ensuring the proper function of the carburettor and fuel system. Here is a detailed guide on how to perform this inspection:
Firstly, it is crucial to understand the function of the float bowl and needle. The float bowl is a reservoir that holds fuel, and the needle is a valve that allows fuel to enter the float bowl when it is opened and prevents fuel from entering when it is closed. The needle is operated by a float, which rises and falls with the fuel level in the float bowl, controlling the needle's position.
To inspect the float bowl and needle, begin by removing the float bowl from the carburettor. This step can be challenging, as the float bowl may be stuck due to fuel residue or corrosion. Using a carburettor cleaner or soaking the area in acetone can help loosen any gummed-up or corroded parts. Be cautious, as some carburettor cleaners are water-based and can worsen corrosion if left for too long. Once the float bowl is removed, inspect it for any signs of damage or leaks.
Next, examine the float and needle assembly. Check if the float moves freely and is not stuck or binding. A simple test for a cracked or pinhole in the float is to shake it and listen for the sound of fuel sloshing around inside, or submerge it in water and look for bubbles. If the float is damaged, it may need to be replaced.
Inspect the needle for any signs of wear or damage. Ensure that the needle can move smoothly up and down and is not stuck in one position. Check if the needle seats properly in its valve seat when the float is in the fully downward position. If the needle does not seat properly, it may need to be adjusted or replaced.
Finally, reassemble the float bowl and needle assembly, ensuring that all parts are clean and free of debris. Test the operation of the carburettor by manually operating the float and needle assembly and observing if the fuel flow stops when the needle is in the closed position.
By following these steps, you can thoroughly inspect the float bowl and needle on a CB750K, ensuring their proper function and maintaining the performance of your motorcycle's fuel system.
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Use a float gauge tool
To check the plastic floats on a CB750K, one method is to use a float gauge tool. This is a highly accurate and durable tool with a thickness of 0.210" (5.3mm) and is set to the Honda specification of 26mm. It is designed to fit the CB750K models from 1969 to 1976.
The float gauge tool can be used to check the float height when the carb is on its side or upside down. The pointed valve section of the tool must be set to just barely "seat" into the brass fitting, or it should stop moving when the float rotates. This ensures the correct level of the floats.
To use the tool effectively, the small brass float tab should be bent slightly so that it just touches the spring-loaded pin on top of the pointed valve section. This will allow for an accurate measurement of the float height.
The float gauge tool is designed to be used with the early carburettor models that have a lip around the bottom of the carburettor body. For CV-type carbs from 1977 onwards, a different tool is required as these carbs do not have a lip and are measured from the gasket surface.
Using a float gauge tool can help ensure the correct float height, which is essential for the proper functioning of the carburettor and fuel system. It allows for a precise measurement and adjustment of the float height, ensuring optimal performance and fuel efficiency.
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Check for faulty needles
To check for faulty needles in your CB750K, there are several steps you can take. Firstly, it is important to inspect the physical condition of the needles. Check that the needles are not bent, as this can cause issues with fuel flow and engine performance. If the needles appear bent, it may be necessary to replace them.
Next, examine the float needles for any signs of wear or damage. The rubber part of the needle should be cone-shaped. If the rubber is noticeably worn or degraded, it is recommended to replace the needles. Additionally, test the spring-loaded side of the needle by pushing it in and ensuring it springs back out easily. If it gets stuck or returns slowly, you can try soaking that end in a carb cleaner to improve its movement.
It is also important to clean the needle seats and ensure there is no build-up of residue or debris. This can be done using a Q-tip and some metal polish or carb cleaner. Give the sides of the needles a quick buff to ensure they don't snag in the bore. Check for any micro-cracks in the carb bowl and overflow needle, as these can also contribute to fuel leaks.
If you are experiencing leaking needles, it may be due to the type of needle used. The early 750 models used metal-tipped float needles, which were prone to leaking. Switching to the later rubber-tipped needles can help resolve this issue. Additionally, check that the float is not getting soaked and losing buoyancy, as this can cause the needle to not seat properly.
Finally, if you are experiencing issues with the jet needle, refer to the factory service manual to ensure you are using the correct type of needle for your model. The manual will specify the correct needle position and provide guidance on adjusting the needle height if needed. It is also important to check the needle jet, as it should always match the needle being used.
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Consider replacement floats
If you are experiencing issues with your Honda CB750's floats, it may be time to consider replacing them. While there are several potential causes for float-related problems, such as gaskets not sealing properly or issues with the needle valve assembly, worn or faulty floats are a common culprit.
One way to diagnose float issues is to manually check their functionality. This can be done by removing the bowl and gently blowing into the fuel barb while manually lifting the float, to observe whether the valve is shutting off the fuel supply. If the floats are not engaging the needle properly, they may need to be replaced.
Another sign that your floats may need replacement is if your motorcycle is leaking fuel or overflowing from the bowl gaskets. This could indicate that the floats are not functioning correctly and are failing to shut off the fuel supply at the appropriate level.
In some cases, issues with the float height adjustment may also warrant float replacement. While the float height specification for the Honda CB750 is typically 26mm, some owners adjust the height to compensate for other running issues or larger jets. However, incorrect float height adjustment can lead to problems, and it may be more prudent to replace the floats with ones that match the specified height.
Additionally, if your floats are original equipment and have been in use for a long period, they may be more prone to failure. Replacing them with newer, more reliable floats can help mitigate the risk of future issues and improve the overall performance of your motorcycle.
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Frequently asked questions
The float height should be 15.5mm, but it is unclear whether this measurement is taken with the carbs lying flat or on their side. One user has suggested that the bike should be upside down, with the float bowl/floats facing up.
One user has suggested making a jig to check the float height off the bike using the clear tube method. Another user has suggested using a 5-gallon bucket to rest the carb rack on.
Some users have reported that their floats started sticking occasionally and would pour gas out of the overflow. This issue was resolved by purchasing cheap replacement plastic floats.
With the bowl off, you should be able to manually close the needle with the float and stop the fuel. If this is successful, the needle is the correct size. If the floats are not shutting off the fuel, you may need to replace the floats.
