Adjusting the high and low jets on a carburetor is a critical step in ensuring optimal engine performance. These adjustments control the air-fuel mixture at idle and high speeds, respectively, and can significantly impact the vehicle’s power, fuel economy, and emissions. Understanding how to adjust these jets correctly is essential for any DIY enthusiast or mechanic.
Before adjusting the carburetor, it’s important to have a basic understanding of its operation. The carburetor mixes air and fuel in a predetermined ratio, which is then drawn into the engine’s cylinders. The high-speed jet controls the fuel flow at high engine speeds, while the low-speed jet regulates the mixture at idle and low speeds. By adjusting the jets, you can fine-tune the air-fuel ratio to match the engine’s specific needs.
The procedure for adjusting the carburetor jets varies depending on the specific model and manufacturer. However, some general guidelines apply. First, locate the high-speed and low-speed jets on the carburetor body. They are typically brass screws with slotted heads. Using a small screwdriver or hex key, gently turn the high-speed jet counterclockwise to lean out the mixture or clockwise to richen it. Make small adjustments (about 1/8 of a turn at a time) and allow the engine to idle for a few seconds after each change. Repeat the process for the low-speed jet, aiming for a smooth and stable idle. Fine-tuning the carburetor jets is a delicate process that requires patience and attention to detail. By following these guidelines and referring to the vehicle’s service manual, you can ensure proper carburetor operation and optimal engine performance.
Understanding Carburetors: Basic Principles
Carburetors are mechanical devices that mix air and fuel in an internal combustion engine. They play a crucial role in ensuring the engine runs smoothly and efficiently.
The basic principle of a carburetor is to control the flow of air and fuel to the engine. As air flows through the carburetor, it creates a Venturi effect, which causes a drop in pressure. This pressure drop draws fuel from a reservoir into the airstream. The mixture of air and fuel is then sent to the engine’s cylinders, where it is ignited.
There are two main types of carburetors: constant-velocity (CV) carburetors and fixed-venturi carburetors. CV carburetors are designed to maintain a constant air velocity through the carburetor, regardless of engine speed. This helps to improve fuel efficiency and engine performance. Fixed-venturi carburetors have a fixed Venturi size, which means that the air velocity through the carburetor will vary with engine speed. This can lead to reduced fuel efficiency and engine performance at low and high speeds.
Carburetors are complex devices, and there are several factors that can affect their performance. These factors include the size of the carburetor, the shape of the Venturi, and the type of fuel being used. It is important to properly adjust the carburetor to ensure that the engine is running at its optimum performance.
Identifying High- and Low-Speed Mixture Screws
Identifying the high- and low-speed mixture screws on a carburetor is crucial for proper adjustment. These screws regulate the fuel-to-air ratio at different engine speeds, ensuring optimal performance and fuel efficiency. Here’s a detailed guide on locating these screws:
2. Identifying the Low-Speed Mixture Screw
The low-speed mixture screw controls the fuel-to-air ratio when the engine is idling or operating at low speeds. It’s typically located near the base of the carburetor, below the throttle plate. Here are some specific tips for identifying it:
- Look for a screw that is usually made of brass or plastic.
- It may have a small spring or a locknut around it.
- Trace the fuel line from the fuel bowl to the carburetor. The low-speed mixture screw is usually located near where the fuel line enters the carburetor.
**Note:** The low-speed mixture screw is sometimes referred to as the idle mixture screw.
| Carburetor Type |
Low-Speed Mixture Screw Location |
| Single-barrel carburetor |
Base of the carburetor, below the throttle plate |
| Two-barrel carburetor |
Base of the primary carburetor, below the throttle plate |
| Four-barrel carburetor |
Base of the primary and secondary carburetors, below the throttle plates |
Strategies for Richening the Mixture
One of the primary methods for adjusting your carburetor is to alter the fuel-to-air mixture. When this adjustment is necessary will be made clear by how your engine performs. If you find that your engine is sputtering or hesitating upon acceleration, it is likely running too lean, meaning there is too much air in the mixture. Conversely, if your engine is running rough or producing black smoke, it is likely running too rich, meaning there is too much fuel in the mixture.
The most straightforward way to richen the mixture is to increase the fuel flow. This can be done by turning the high-speed mixture screw clockwise. However, this adjustment should be made gradually, as even a slight change can have a significant impact on the engine’s performance.
Another way to richen the mixture is to decrease the airflow. This can be done by partially closing the choke, which will reduce the amount of air entering the carburetor. However, this adjustment should be used only as a temporary measure, as it can lead to problems with starting the engine and can cause the engine to run too hot.
Fine-tuning the Mixture: Idle Mixture Screw
The idle mixture screw also plays a role in fuel-to-air ratio. Adjusting this screw is one of the final carburetor adjustments you will make, generally after addressing issues with the high-speed mixture. The engine should be fully warmed up before attempting to adjust the idle mixture screw, and you should have already adjusted the idle speed so that it is between 750 and 850 RPM. To properly adjust the idle mixture screw, set the parking brake, place the transmission in park, or set the clutch pedal to neutral for vehicles with a manual transmission. Then, turn the idle mixture screw clockwise until the idle quality noticeably worsens, then back out counterclockwise until idle quality worsens in the opposite direction. Now, turn the idle mixture screw back clockwise to the halfway point between those two settings.
Here is a helpful table with the effects of turning the idle mixture screw in either direction:
| Turning Idle Mixture Screw Clockwise |
Turning Idle Mixture Screw Counterclockwise |
| Leaner mixture |
Richer mixture |
| Increased intake vacuum |
Decreased intake vacuum |
| Higher idle speed |
Lower idle speed |
| Possible hesitation upon acceleration |
Possible black exhaust smoke |
Techniques for Leaning Out the Mixture
Identifying When the Mixture Is Too Rich
Signs that the mixture is excessively rich can include:
- Black smoke from the exhaust
- Fouled spark plugs
- Poor engine performance
Adjusting the Mixture Screws
Most carburetors have one or two mixture screws that allow for fine-tuning the air-fuel ratio. Turning the screws clockwise leans out the mixture, while counterclockwise enriches it.
Synchronizing Multiple Carburetors
If a vehicle has multiple carburetors, it’s crucial to ensure they are synchronized. Misalignment can lead to an uneven distribution of air and fuel, affecting engine performance.
Using a Vacuum Gauge
A vacuum gauge is a valuable tool for fine-tuning the carburetor mixture. It measures the pressure in the intake manifold, which changes depending on the air-fuel ratio. A stable vacuum reading indicates an optimal mixture.
| Vacuum Reading |
Air-Fuel Ratio |
| < 15 inches Hg |
Too rich |
| 16-20 inches Hg |
Ideal |
| > 20 inches Hg |
Too lean |
Fine-Tuning the Idle Speed Screw
Once the carburetor is adjusted for maximum performance, the idle speed can be fine-tuned. This is done by adjusting the idle speed screw, which is typically located on the side of the carburetor. Turning the screw clockwise will increase the idle speed, while turning it counterclockwise will decrease the idle speed.
The ideal idle speed will vary depending on the engine, but a good rule of thumb is to set it so that the engine idles smoothly without stalling or surging.
To fine-tune the idle speed, start by turning the screw clockwise until the engine starts to idle roughly. Then, turn the screw counterclockwise until the engine idles smoothly. If the engine stalls, turn the screw clockwise until the engine idles again.
Once the idle speed is set, check the air-fuel mixture by reading the spark plugs. The spark plugs should be a light tan color. If they are black, the mixture is too rich and the idle speed screw needs to be turned counterclockwise. If they are white, the mixture is too lean and the idle speed screw needs to be turned clockwise.
| Spark Plug Color |
Air-Fuel Mixture |
Idle Speed Screw Adjustment |
| Light tan |
Correct |
No adjustment needed |
| Black |
Too rich |
Turn idle speed screw counterclockwise |
| White |
Too lean |
Turn idle speed screw clockwise |
Impact of Adjustments on Engine Performance
Adjusting the high and low jets on a carburetor significantly impacts engine performance by affecting the air-fuel mixture entering the combustion chamber. Here’s how these adjustments affect various engine characteristics:
Low-Speed Mixture
Adjusting the low jet (idle mixture) affects the air-fuel ratio at low engine speeds, primarily during idle and slow acceleration. When the low jet is turned in (richer), the mixture contains more fuel, resulting in a smoother idle and improved acceleration. Conversely, turning out (leaner) reduces fuel flow, leading to a leaner mixture with a higher idling speed and less responsive acceleration.
Mid-Speed Mixture
Adjustments to the mid-speed circuit (typically via a jet or emulsion tube) influence the air-fuel ratio at moderate engine speeds, such as during cruising. A richer mid-range mixture provides better throttle response and torque, while a leaner mixture improves fuel economy and reduces emissions.
High-Speed Mixture
The high jet (main jet) controls the air-fuel ratio at higher engine speeds, where the engine’s airflow demand is highest. A richer high-speed mixture provides better power, but can increase fuel consumption and promote fouling of spark plugs. A leaner mixture improves fuel economy and reduces spark plug fouling, but may reduce power.
Impact on Engine Temperature
Carburetor adjustments can also affect engine temperature. A richer mixture generally runs cooler due to the increased fuel acting as a cooling agent. Conversely, a leaner mixture causes higher cylinder head temperatures, which can lead to detonation (knock) and potential engine damage if not addressed.
Impact on Acceleration
Proper carburetor adjustments improve engine acceleration by ensuring a smooth transition of the air-fuel mixture as engine speed increases. A richer mixture provides better acceleration at low speeds, while a leaner mixture enhances acceleration at higher speeds.
Fuel Consumption
Carburetor adjustments can influence fuel consumption. A leaner mixture generally improves fuel efficiency, while a richer mixture typically increases fuel consumption due to the increased fuel flow. Finding the optimal balance between performance and fuel economy is crucial.
| Adjustment |
Impact on Mixture |
Impact on Performance |
| Low Jet (Idle Mixture) |
Controls air-fuel ratio at low engine speeds |
Affects idle smoothness and acceleration |
| Mid-Speed Circuit |
Influences air-fuel ratio at moderate engine speeds |
Improves throttle response, torque, and fuel economy |
| High Jet (Main Jet) |
Controls air-fuel ratio at high engine speeds |
Affects power, fuel consumption, and spark plug fouling |
Indicators of a Properly Adjusted Carburetor
A properly adjusted carburetor ensures optimal engine performance, fuel efficiency, and reduced emissions. Here are some key indicators:
1. Smooth Idle
The engine idles smoothly and consistently without any surging or stalling.
2. Responsive Acceleration
The engine accelerates quickly and smoothly without any hesitation or bogging down.
3. Proper Air-Fuel Mixture
The exhaust gases appear clean and colorless, indicating a balanced air-fuel mixture.
4. Improved Fuel Efficiency
The vehicle achieves optimal fuel consumption due to the efficient combustion of air and fuel.
5. Reduced Emissions
Proper carburetor adjustment minimizes harmful emissions, such as hydrocarbons, carbon monoxide, and nitrogen oxides.
6. Extended Engine Life
A well-adjusted carburetor ensures that the engine receives the correct amount of fuel, reducing wear and tear on engine components.
7. How to Adjust High and Low Speeds on a Carburetor
Step 1: Warm up the engine. Allow the engine to run at idle speed until it reaches operating temperature.
Step 2: Locate the idle mixture screw. This screw typically has a spring on top and is located on the side of the carburetor. It controls the amount of air mixed with fuel at idle.
Step 3: Adjust the idle mixture screw. Use a small screwdriver to turn the screw in or out. Turning it in will lean out the mixture, while turning it out will enrich the mixture.
Step 4: Monitor engine idle speed. As you adjust the screw, the engine’s idle speed will change. Aim for an idle speed of around 800-1000 RPM.
Step 5: Adjust the main jet. The main jet controls the amount of fuel mixed with air at higher speeds. It is located in the center of the carburetor. Use a wrench to remove the main jet and select a larger jet for a richer mixture or a smaller jet for a leaner mixture.
Step 6: Fine-tune the adjustment. Start with a test drive and observe engine performance. Make small adjustments as needed to achieve optimal acceleration and fuel efficiency.
How to Adjust High and Low Screws on a Carburetor
Carburetors are a critical component of older vehicles, controlling the air-fuel mixture entering the engine. Adjusting the high and low screws plays a vital role in ensuring optimal performance and fuel efficiency.
Common Pitfalls and Troubleshooting
Engine Won’t Start
If your engine won’t start after adjusting the screws, double-check the fuel supply, spark plugs, and electrical connections. Ensure there are no loose or disconnected components.
Rough Idle
If the engine idles rough, adjust the low screw slightly until the idle smooths out. Be cautious not to overtighten, as this can lean out the mixture too much.
Hesitation During Acceleration
Hesitation during acceleration may indicate a lean high-speed mixture. Turn the high screw out slightly to richen the mixture and improve throttle response.
Stalling When Throttling Up
Stalling when throttling up can be caused by a rich low-speed mixture. Adjust the low screw in slightly to lean out the mixture and resolve the issue.
Poor Fuel Economy
If you notice a drop in fuel economy, adjust the screws to achieve a more optimal air-fuel ratio. A properly tuned carburetor will maximize fuel efficiency.
Engine Surging or Bucking
Surging or bucking can be caused by an unbalanced air-fuel mixture. Fine-tune both the high and low screws to find the ideal balance and eliminate the issue.
Black or White Smoke from Exhaust
Black smoke indicates a rich mixture, while white smoke can indicate a lean mixture. Adjust the screws accordingly to rectify the issue and maintain proper combustion.
Troubleshooting Table
| Symptom |
Cause |
Solution |
| Engine won’t start |
Fuel supply or electrical issues |
Inspect fuel system and electrical connections |
| Rough idle |
Lean low-speed mixture |
Adjust low screw out (richer) |
| Hesitation during acceleration |
Lean high-speed mixture |
Adjust high screw out (richer) |
| Stalling when throttling up |
Rich low-speed mixture |
Adjust low screw in (leaner) |
| Poor fuel economy |
Suboptimal air-fuel ratio |
Fine-tune both screws for optimal balance |
Safety Precautions When Adjusting Carburetors
Before adjusting any carburetor, it is crucial to follow these safety precautions to ensure your safety and prevent any potential hazards.
- Disconnect Battery: Disconnect the negative terminal of the battery to prevent electrical shocks.
- Remove Air Filter: Take off the air filter to access the carburetor.
- Use Protective Gear: Wear eye protection and gloves to shield yourself from any fuel or debris.
- Avoid Smoking or Naked Flames: Carburetors deal with flammable materials, so refrain from smoking or using naked flames nearby.
- Keep Area Ventilated: Work in a well-ventilated area to prevent inhaling harmful fumes.
- Handle Fuel Carefully: Avoid spilling fuel and keep it away from open flames or heat sources.
- Use Proper Tools: Only use tools designed for carburetor adjustments to prevent damage.
- Avoid Overtightening: Never overtighten any nuts or bolts to prevent damaging the carburetor.
- Consult a Professional: If you encounter difficulties or uncertainties, seek assistance from a qualified mechanic.
By adhering to these precautions, you can safely adjust your carburetor and ensure optimal engine performance.
9. Adjust the High- and Low-Speed Mixture Screws
a) High-Speed Mixture Screw:
This screw controls the fuel-air mixture at wide open throttle (WOT).
– Turn the screw clockwise to lean out the mixture.
– Turn the screw counterclockwise to enrich the mixture.
– Aim for a slight hesitation upon rapid acceleration followed by smooth power delivery.
b) Low-Speed Mixture Screw:
This screw regulates the fuel-air mixture at idle and low throttle openings.
– Turn the screw clockwise to lean out the mixture.
– Turn the screw counterclockwise to enrich the mixture.
– Adjust for a smooth idle and minimal throttle response.
Note: These adjustments should be made incrementally, with the engine running and warmed up. Use the following table as a reference:
| Adjustment |
Effect |
| High-Speed Mixture Screw, Clockwise |
Leans mixture, reducing fuel flow |
| High-Speed Mixture Screw, Counterclockwise |
Enriches mixture, increasing fuel flow |
| Low-Speed Mixture Screw, Clockwise |
Leans mixture, reducing idle speed |
| Low-Speed Mixture Screw, Counterclockwise |
Enriches mixture, increasing idle speed |
Importance of Regular Carburetor Maintenance
The carburetor is a crucial component in regulating the air-fuel mixture in a gasoline engine. By providing the engine with an optimal blend of air and fuel, the carburetor ensures efficient combustion, smooth operation, and optimal performance. However, like any other engine component, carburetors require regular maintenance to maintain their functionality and prevent issues. Here’s why carburetor maintenance is essential:
Improved Engine Performance: A well-maintained carburetor ensures that the engine receives the correct air-fuel ratio, leading to improved combustion, smoother running, and increased power and torque.
Fuel Efficiency: A carburetor in good condition delivers the precise amount of fuel needed by the engine, resulting in optimal fuel economy and reduced fuel consumption.
Reduced Emissions: A properly functioning carburetor helps the engine burn fuel more efficiently, leading to reduced emissions of pollutants such as hydrocarbons, carbon monoxide, and nitrogen oxides.
Extended Engine Life: Regular carburetor maintenance helps prevent premature engine wear and tear by ensuring the engine is running efficiently without excessive fuel dilution in the oil.
Preventing Carburetor Problems: Regular maintenance can identify and address minor issues before they develop into major problems, such as clogging, sticking, or leaking, which can lead to engine performance issues or even breakdowns.
Moreover, carburetor maintenance involves adjustments to the idle mixture and speed, which play a vital role in optimizing the engine’s performance and fuel efficiency. Here’s how to perform these adjustments:
Adjusting the Idle Mixture
The idle mixture adjustment screw regulates the carburetor’s air-fuel ratio at idle. Turning the screw clockwise leans out the mixture (more air, less fuel), while turning it counterclockwise enriches the mixture (less air, more fuel).
To adjust the idle mixture, follow these steps:
- Start the engine and let it warm up to operating temperature.
- Locate the idle mixture adjustment screw on the carburetor, typically a brass or plastic screw with a flathead or pointed tip.
- Using a screwdriver, turn the screw slowly in small increments (1/8 to 1/4 turn at a time).
- Observe the engine response. If the engine runs smoother, has improved idle stability, and does not stall, you have adjusted the mixture correctly.
- Fine-tune the adjustment until you achieve the best idle quality and response.
Note: If the engine stalls or runs rough during the adjustment, turn the screw in the opposite direction to enrich the mixture.
Adjusting the Idle Speed
The idle speed adjustment screw sets the engine’s idle speed. Turning the screw clockwise increases the idle speed, while turning it counterclockwise decreases the idle speed.
To adjust the idle speed, follow these steps:
- Locate the idle speed adjustment screw, usually a plastic or brass knob or lever.
- Using the knob or lever, turn the screw in small increments to increase or decrease the idle speed.
- Observe the engine’s idle speed on the tachometer or by listening to the idle.
- Adjust the speed to the manufacturer’s recommended specifications or to your preference.
- Fine-tune the adjustment until you achieve the desired idle speed and stability.
Note: The ideal idle speed varies depending on the engine and vehicle. Refer to the manufacturer’s specifications or consult with a qualified mechanic for the recommended idle speed.
How to Adjust High and Low on Carburetor
Adjusting the high and low on a carburetor is a simple task that can be performed with a few basic tools. By following these steps, you can ensure that your carburetor is properly tuned and your engine is running at its best.
Adjusting the High
1. Start by warming up the engine to operating temperature.
2. Locate the high speed adjustment screw on the carburetor. It is usually located on the top of the carburetor, near the throttle linkage.
3. Turn the high speed adjustment screw clockwise to increase the idle speed, or counterclockwise to decrease the idle speed.
4. Adjust the screw until the engine idles at the desired speed.
Adjusting the Low
1. Locate the low speed adjustment screw on the carburetor. It is usually located on the bottom of the carburetor, near the fuel inlet.
2. Turn the low speed adjustment screw clockwise to increase the low speed mixture, or counterclockwise to decrease the low speed mixture.
3. Adjust the screw until the engine idles smoothly and without hesitation.
Tips
It is important to make small adjustments to the high and low screws when tuning the carburetor. Turning the screws too far in either direction can cause the engine to run poorly or even stall.
If you are unsure about how to adjust the carburetor, it is best to consult a mechanic.
People Also Ask
How do I know if my carburetor needs adjusting?
There are a few signs that your carburetor may need adjusting, including:
- Rough idling
- Stalling
- Poor fuel economy
- Black smoke from the exhaust
How often should I adjust my carburetor?
The frequency with which you need to adjust your carburetor will vary depending on how often you drive your vehicle and the conditions in which you drive it. However, it is generally recommended to adjust the carburetor every few thousand miles.
Can I adjust the carburetor myself?
Yes, it is possible to adjust the carburetor yourself. However, it is important to follow the instructions carefully and to make small adjustments. If you are unsure about how to adjust the carburetor, it is best to consult a mechanic.
