Adjusting Fuel Ratio In Kz750: A Step-By-Step Guide

The Kawasaki KZ750 is a motorcycle first offered in 1976, praised for its simplicity and ease of maintenance. If you're looking to change the fuel ratio on your KZ750, you may be considering a fuel injection conversion, or rejetting the carbs. Fuel injection kits are available, but they are expensive and require a lot of time and effort to install. Rejetting the carbs is a more popular option, and can be done by a dealership or at home with a forum guide. Dynojet Carburetor Jet Kits are also available, and provide a simple and affordable solution to carburetion problems.

Adjusting the fuel/air mixture

Adjusting the fuel-air mixture on a KZ750 motorcycle can be done by making changes to the jetting in the carburetor. This process involves adjusting the amount of fuel and air that enters the engine to ensure the correct fuel-air mixture for optimal performance. Here is a step-by-step guide to help you adjust the fuel-air mixture on your KZ750:

Step 1: Understand the Basics

Before making any adjustments, it is important to understand the components involved in the fuel-air mixture process. The main components are the air filter, carburetor, and exhaust system. The air filter allows air to enter the engine, the carburetor mixes the air with fuel, and the exhaust system releases the burnt fuel-air mixture after combustion. Any modifications to these components, such as installing a new air filter or exhaust, can affect the fuel-air mixture and may require adjustments to the jetting.

Step 2: Assess the Need for Adjustment

Signs that your KZ750's fuel-air mixture may need adjustment include bogging down when accelerating, backfiring, increased fuel consumption, or a loss of power. A proper fuel-air mixture should result in a light chocolate brown colour on the spark plugs. If the spark plugs appear grey or white, the mixture is too lean; if they are dark or sooty, the mixture is too rich.

Step 3: Adjust the Fuel Screws

Locate the fuel screws on the sides of the carburetors. These screws control the fuel-air mixture at idle. To richen the mixture, back out the screws (turn them counterclockwise) by about two turns from the fully seated position. This will increase the fuel flow during idle and can help with a lean mixture.

Step 4: Adjust the Needle Height or Clip Position

If adjusting the fuel screws does not resolve the issue, the next step is to adjust the needle height or clip position. This will affect the fuel-air mixture at higher RPMs. To richen the mixture, raise the needle slightly or move the clip closer to the point of the needle. This will allow more fuel to enter the needle jet, resulting in a richer mixture.

Step 5: Change the Jets

If the above adjustments do not resolve the issue, you may need to change the jets in the carburetors. The jets are small cylinders that control the amount of fuel entering the engine. You may need to change to a larger or smaller jet size, depending on whether you need to increase or decrease the fuel flow. Changing the jets is usually done after modifying the exhaust or air intake, as these modifications can significantly affect the fuel-air mixture.

Step 6: Seek Professional Help

If you are unsure about any of the above steps or if your adjustments do not improve the performance of your KZ750, it is recommended to seek the help of a professional mechanic or a dealership. They will have the experience and tools to properly diagnose and adjust the fuel-air mixture to ensure optimal performance and safety.

Carburetor jet kits

Dynojet Carburetor Jet Kits, for example, come with comprehensive instructions that show step-by-step installation and testing procedures to ensure a perfect setup every time. They also offer free telephone and internet support to their customers.

Some companies, such as CarbJetKits, offer a jet kit calculator on their website to help customers determine the correct jets for their specific setup. This can make the installation process much easier, especially for those who are not familiar with motorcycle carbs.

Overall, carburetor jet kits can be a great way to improve the performance and fuel efficiency of your vehicle while also enhancing the smoothness and ease of your ride.

Fuel injection conversion

If you're considering converting your carbureted KZ750 to fuel injection, there are a few things you should know. Firstly, it's important to do your research before starting such a complex project. The MegaManual is a good resource to understand everything about EFI conversions.

Converting to EFI is not a trivial task and requires a significant amount of time, effort, and money. It involves a lot more work than simply rebuilding the carburetors and servicing the bike. You'll need to decide on the right throttle bodies for your KZ750. The GPz1100 and 750 Turbo are the only direct-fit throttle bodies, and using parts from these older models might not be the best idea due to their age.

You could also consider modern throttle bodies from other Kawasaki or Honda models, but you'll need to ensure the diameter and spacing are correct. Keep in mind that any EFI conversion will require careful calibration to get it just right, and there might not be a noticeable performance improvement over the stock carbs.

Alternatively, you could look into carburetor jet kits, which can provide a simple and affordable solution to carburetion problems. These kits can increase power and smoothness throughout the rev range while maintaining fuel economy. They also come with comprehensive instructions and support, making them a more straightforward option than a full EFI conversion.

Jetting the carbs

To jet the carbs on your KZ750, you'll need to make adjustments to the jets, which control the amount of fuel and air that enters the engine. This process can be complex and it's recommended to consult a professional or a forum for specific instructions for your bike. However, here is a general guide on jetting the carbs:

First, it's important to understand the different types of jets in your carburetor:

• Main Jets: These control the amount of fuel delivered at full throttle.
• Pilot Jets: These control the amount of fuel delivered at idle and low speeds.
• Needle Jets: These control the fuel and air mixture at partial throttle.
• Air Jets: These control the amount of air that mixes with the fuel.

When making changes to your carburetor, it's important to make small adjustments and test the bike after each change to see the effects. You can assess the results by observing the colour of the spark plugs or using a tool like a Colortune.

If you're experiencing bogginess or poor performance after making changes to your air intake or exhaust, you'll likely need to adjust your jets to compensate for the changed airflow. For example, if you've installed a performance air filter, you may need to increase the size of your main jets to allow more fuel into the engine. On the other hand, if you've installed a more restrictive air filter, you may need to decrease the size of your main jets.

It's also important to ensure your carburetor is clean and in good condition before making any adjustments. A dirty or malfunctioning carburetor can cause poor performance and fuel economy.

Additionally, you can use a Dynojet Kit, which provides a simple and affordable solution to carburetion problems. These kits come with comprehensive instructions and offer telephone and internet support for installation. They are designed to increase power and smoothness throughout the entire rev range while maintaining optimum fuel economy.

When adjusting your jets, it's crucial to make small changes and test the bike after each adjustment to avoid making the mixture too lean or too rich, which can damage your engine. Always refer to a professional or a trusted forum for specific advice on jetting your KZ750's carbs.

Idle adjustment

The idle speed of a KZ750 motorcycle can be adjusted by turning the idle screw, which is located between the second and third carburetors. The idle speed should be set at around 1000 RPM for a 1979 750 twin, and 1300 RPM for an 81 KZ750 LTD.

To adjust the idle speed, the engine should be fully warmed up. Start the bike with the choke on, and as the engine warms up, gradually reduce the choke. The choke should be fully off after about 5 minutes of riding. If the idle speed is still too high, there may be a problem with the throttle cable, such as a tight or kinked cable, or a lack of lubrication.

Another potential issue could be an air leak around the carburetors. This can be tested by spraying WD-40 or starter fluid around the carburetors; if there is a leak, the engine will rev. Before attempting to adjust the idle speed, it is important to ensure that the fuel tank, petcock, and carburetors are clean, and that the fuel levels in the bowls are correctly set.

Frequently asked questions