How To Calibrate A Carburetor

Your car’s carburetor is a vital component that mixes air and fuel to create the combustible mixture that powers your engine. Over time, the carburetor can become out of adjustment, which can lead to a variety of problems, including poor performance, increased fuel consumption, and even engine damage. That’s why it’s important to know how to calibrate a carburetor correctly. Calibrating a carburetor is not a difficult task, but it does require some basic tools and knowledge. With a little patience, however, you can save yourself the time and expense of taking your car to a mechanic.

The first step in calibrating a carburetor is to identify the type of carburetor you have. There are two main types of carburetors: single-barrel and two-barrel. Single-barrel carburetors are the most common type, and they are found on most older cars and some newer models as well. Two-barrel carburetors are more complex than single-barrel carburetors, and they are found on some newer cars and trucks. Once you have identified the type of carburetor you have, you can begin the calibration process.

To calibrate a single-barrel carburetor, you will need a screwdriver, a wrench, and a vacuum gauge. First, locate the idle speed screw on the carburetor. This screw is usually located on the side of the carburetor, and it is typically labeled “Idle Speed” or “ISC.” Turn the idle speed screw clockwise to increase the idle speed or counterclockwise to decrease the idle speed. Next, locate the air/fuel mixture screw on the carburetor. This screw is usually located on the bottom of the carburetor, and it is typically labeled “A/F” or “Mixture.” Turn the air/fuel mixture screw clockwise to lean out the mixture or counterclockwise to richen the mixture. Finally, connect the vacuum gauge to the carburetor and start the engine. Adjust the idle speed screw and the air/fuel mixture screw until the vacuum gauge reading is within the manufacturer’s specifications.

Understanding Carburetor Basics

A carburetor is a crucial component in the fuel system of an internal combustion engine, responsible for mixing the correct ratio of fuel and air for optimal engine performance. Understanding the basics of carburetor operation is essential for proper calibration and maintenance.

Venturi Effect

The core principle behind a carburetor’s operation lies in the Venturi effect. As air flows through a Venturi-shaped tube, its velocity increases while its pressure decreases. This reduced pressure creates a suction force that draws fuel from the carburetor’s fuel bowl. The amount of fuel drawn in is directly proportional to the air velocity, ensuring a precise air-fuel mixture for combustion.

Float Bowl

The float bowl is a chamber that serves as a reservoir for fuel. A float mechanism regulates the fuel level within the bowl, maintaining a constant head pressure for consistent fuel delivery. As the fuel level drops, the float descends and opens a valve, allowing more fuel to flow into the bowl. Conversely, when the fuel level rises, the float ascends and closes the valve, preventing overfilling.

Components of the Float Bowl:

Component	Function
Float	Regulates fuel level by controlling the fuel inlet valve
Fuel Inlet Valve	Allows fuel to flow into the bowl when the fuel level drops
Fuel Outlet	Provides a constant head pressure of fuel for the carburetor
Drain Plug	Allows for draining of excess fuel or water

Tools and Materials Required

To perform carburetor calibration, you will require the following:

* Screwdrivers (flathead and Phillips)
* Wrenches (various sizes)
* Socket set
* Carburetor cleaner
* Rags
* Fuel filter
* Replacement gaskets (optional)

Carburetor cleaner

A carburetor cleaner is a solvent used to dissolve and remove dirt, carbon, and other deposits from the internal components of the carburetor. It is typically available in aerosol cans or bulk containers. When selecting a carburetor cleaner, choose one that is specifically designed for automotive use. Some carburetor cleaners contain harsh chemicals that can damage the carburetor if used improperly.

Screwdrivers

A variety of screwdrivers will be needed to remove and adjust the various screws and bolts on the carburetor. Common screwdriver types include flathead, Phillips, and Torx.

Wrenches

A set of wrenches will be needed to remove and tighten nuts and bolts on the carburetor and its associated components. The specific sizes of wrenches required will depend on the make and model of the vehicle.

Socket set

A socket set is a collection of sockets and a ratchet or impact wrench. Sockets are used to remove and tighten nuts and bolts that are recessed or difficult to reach with a standard wrench.

Fuel filter

A new fuel filter should be installed whenever the carburetor is serviced. The fuel filter removes impurities from the fuel, preventing them from reaching the carburetor and causing problems.

Replacement gaskets

If the old carburetor gaskets are damaged or worn, they should be replaced with new ones. Gaskets help to seal the carburetor and prevent leaks.

Preparing Your Carburetor

Before adjusting the carburetor, it’s important to prepare it properly. This involves cleaning and inspecting the carburetor to ensure it’s in good working condition. Here’s a step-by-step guide to preparing your carburetor:

1. Gather materials

You’ll need the following materials:

Item	Purpose
Carburetor cleaner	To clean the carburetor
Compressed air	To blow out any dirt or debris
Screwdrivers	To remove and adjust carburetor components
Wrenches	To loosen and tighten nuts and bolts

2. Remove the carburetor

Follow these steps to remove the carburetor:

Disconnect the fuel line and throttle linkage.
Remove the bolts holding the carburetor to the intake manifold.
Carefully lift the carburetor off the intake manifold.

3. Clean the carburetor

Use carburetor cleaner to clean the carburetor thoroughly. Be sure to spray all the passages and jets. Once you’ve cleaned the carburetor, blow it out with compressed air to remove any remaining cleaner or debris. Inspect the carburetor carefully for any damage or wear. If you find any issues, it’s important to address them before proceeding with the calibration.

Here are some additional tips for cleaning the carburetor:

Don’t use a wire brush to clean the carburetor, as this can damage the delicate components.
Be careful not to get any carburetor cleaner on the electrical components.
Allow the carburetor to dry completely before reinstalling it.

Adjusting the Idle Mixture Screw

The idle mixture screw controls the air-fuel ratio at idle speed. Adjusting this screw is a critical step in carburetor calibration, as it directly affects the smoothness and stability of your engine’s idle.

Step 1: Warm Up the Engine

Start the engine and let it run at idle for several minutes to warm it up. A warm engine provides more accurate results for carburetor adjustments.

Step 2: Locate the Idle Mixture Screw

On most carburetors, the idle mixture screw is located on the side or bottom of the carburetor body. It is typically a small screw with a spring-loaded head.

Step 3: Adjust the Screw

Slowly turn the idle mixture screw in or out while observing the engine’s idle speed and exhaust color. Generally, turning the screw clockwise (out) will lean out the mixture, while turning it counterclockwise (in) will enrich the mixture.

Step 4: Tune for Best Idle

Adjust the idle mixture screw until you achieve the following conditions:

Characteristic	Optimal Setting
Engine Speed	Stable and smooth idle around 750-900 RPM
Exhaust Color	Slightly grayish or white (lean) to slightly black or blue (rich)
Acceleration Response	Crisp and immediate throttle response upon acceleration

Verifying Fuel Level in the Float Bowl

After the float drop, a visual inspection of the fuel level in the float bowl is necessary to ensure proper fuel delivery. With the engine running, observe the fuel level through the sight glass or viewing window on the carburetor. The fuel level should be within the specified range, typically between 1/4 to 1/2 inch below the top of the bowl.

If the fuel level is too low, it can lead to lean running conditions, resulting in poor performance and potential engine damage. Conversely, if the fuel level is too high, it can cause flooding, resulting in difficulty starting and rough idling.

To adjust the fuel level, bend the float tab up or down to raise or lower the float, respectively. Make small adjustments at a time and recheck the fuel level after each adjustment until the desired level is achieved. Note that different carburetors have different methods of adjusting the float level, so refer to the manufacturer’s specifications for specific instructions.

Fuel Level Range	Typical Sight Glass Position
1/4 to 1/2 inch below top of bowl	1/4 to 1/2 inch above bottom of window

Setting the Throttle Position Sensor

To ensure the most accurate calibration, the throttle position sensor (TPS) needs to be set correctly. This critical step involves meticulously following these guidelines:

1. Start by ensuring the throttle body is clean and free of any obstruction that may affect sensor readings.

2. Locate the TPS on the throttle body and identify the screw that secures it in place.

3. Loosen the screw without fully removing it, allowing some play in the sensor’s position.

4. Connect a digital multimeter to the TPS signal wire (typically a yellow wire) and ground.

5. With the key turned to the “On” position, the multimeter should read around 0.5 volts when the throttle is fully closed.

6. Adjust the TPS position by carefully turning it clockwise or counterclockwise while monitoring the multimeter reading. The goal is to achieve a voltage value of 4.5 volts when the throttle is fully open. Perform this adjustment incrementally, in small and precise movements, to obtain the desired voltage value without overshooting.

| Voltage Range | Throttle Position |
|—|—|
| 0.5 – 1 V | Fully closed (idle) |
| 2 – 3 V | Part throttle |
| 4 – 4.5 V | Fully open (wide open throttle) |

7. Once the voltage reading is correct, tighten the TPS screw to secure its position and ensure it remains stable.

8. Verify the TPS calibration by cycling the throttle through its full range (closed to wide open) and confirming that the voltage readings correspond to the table above.

By meticulously following these steps, you can ensure that the throttle position sensor is calibrated correctly, providing the engine with accurate information about the throttle opening, leading to optimal engine performance and fuel efficiency.

Calibrating the Air-Fuel Ratio

Idle Mixture Screw Adjustment

1. Start the engine and let it idle.

2. Turn the idle mixture screw slowly clockwise until the engine speed just starts to drop.

3. Then, turn it counterclockwise until the engine speed just starts to increase.

4. Adjust the screw back and forth between these two points until the engine speed is at its highest.

Main Jet Selection

1. Determine the appropriate main jet size for your engine size and configuration.

2. Install the main jet in the carburetor.

3. Test ride the motorcycle and observe the engine performance.

4. If the engine is running lean (hesitates or boggs down under acceleration), increase the main jet size.

5. If the engine is running rich (smokes or fouls spark plugs), decrease the main jet size.

Needle Jet Calibration

1. Select the appropriate needle jet for your engine size and configuration.

2. Install the needle jet in the carburetor.

3. Test ride the motorcycle and observe the engine performance.

4. If the engine is running lean at mid-range (hesitates or boggs down), raise the needle jet by one clip position.

5. If the engine is running rich at mid-range (smokes or fouls spark plugs), lower the needle jet by one clip position.

Pilot Jet Selection

1. Determine the appropriate pilot jet size for your engine size and configuration.

2. Install the pilot jet in the carburetor.

3. Test ride the motorcycle and observe the engine performance.

4. If the engine is running lean at idle or low throttle (hesitates or boggs down), increase the pilot jet size.

5. If the engine is running rich at idle or low throttle (smokes or fouls spark plugs), decrease the pilot jet size.

Starter Jet Adjustment

1. Install the starter jet in the carburetor.

2. Test ride the motorcycle and observe the engine performance.

3. If the engine is hard to start or hesitates on acceleration, increase the starter jet size.

4. If the engine is flooding or running rich on cold starts, decrease the starter jet size.

Accelerator Pump Adjustment

1. Adjust the accelerator pump so that it delivers a small squirt of fuel when the throttle is opened.

2. Test ride the motorcycle and observe the engine performance.

3. If the engine hesitates or bogs down on acceleration, increase the accelerator pump volume.

4. If the engine smokes or runs rich on acceleration, decrease the accelerator pump volume.

Float Level Adjustment

Float Level Table

Carburetor Model	Float Level
Keihin CV	12-14mm
Mikuni BST	14-16mm
Mikuni TM	7.5-9.5mm

1. Measure the distance from the carburetor bowl gasket surface to the top of the float.

2. Adjust the float level to the specified value.

3. Test ride the motorcycle and observe the engine performance.

4. If the engine is running lean (hesitates or boggs down), lower the float level.

5. If the engine is running rich (smokes or fouls spark plugs), raise the float level.

Checking for Leaks and Correcting Issues

Before adjusting the carburetor, it’s crucial to ensure that there are no leaks. Leaks can introduce air into the system, affecting the fuel-air mixture and causing engine performance issues.

Checking for Leaks

To check for leaks, follow these steps:

Inspect the carburetor visually for any visible leaks or cracks.
Start the engine and let it idle.
Spray carburetor cleaner around the following areas:
- Base gasket
- Throttle shaft
- Vacuum lines
- Fuel lines
If you notice any changes in engine speed or performance, it indicates a leak.

Correcting Leaks

If you find any leaks, you need to correct them before proceeding with carburetor calibration. Common methods include:

Tightening bolts and screws: Check and tighten all bolts and screws on the carburetor body and fuel lines.
Using sealants: Apply a thin layer of sealant around the base gasket or throttle shaft to prevent leaks.
Replacing gaskets or lines: If the gaskets or fuel lines are damaged or worn, replace them with new ones.
Checking vacuum lines: Ensure all vacuum lines are connected securely and free from cracks or damage. Replace any faulty lines.

Component	Common Issue	Solution
Base Gasket	Leak between carburetor and manifold	Tighten bolts, apply sealant
Throttle Shaft	Air leak around throttle shaft	Tighten shaft, apply sealant
Vacuum Lines	Leak in vacuum system	Check connections, replace faulty lines
Fuel Lines	Fuel leaking from lines	Inspect and replace damaged lines

Checking Ignition Timing

Igniting your car is a complex process that takes place within the combustion chamber of an internal combustion engine. The timing of this process is crucial for several reasons. The combustion process generates a lot of heat as a byproduct of the chemical reaction between fuel molecules and oxygen. If this heat is not released at the correct instant, it can lead to overheating and damage to the engine. It can also result in decreased fuel efficiency and increased emissions.

A spark plug is the critical component that initiates combustion in an engine. The spark plug is connected to a high-voltage ignition coil and receives a brief pulse of electricity at regular intervals. This electrical current creates a spark that ignites the air-fuel mixture within the combustion chamber, which is comprised of vaporized fuel and air. Adequate timing is necessary for when the spark is generated, just before the piston reaches the top of its compression stroke. The ideal timing is determined by the engine’s design and the type of fuel used.

The ignition timing can be adjusted by modifying the position of the distributor within the engine. The distributor is responsible for distributing a portion of the current from the ignition coil to each spark plug. By rotating the distributor, the timing of each spark can be adjusted. The ignition timing is typically checked when the engine is idling and set according to the specifications provided by the vehicle manufacturer. A timing light can be used to check the timing more precisely.

Symptoms of Incorrect Ignition Timing	Effects of Incorrect Timing
Engine knocking or “pinging”	Premature wear on engine components
Difficulty starting the engine	Increased fuel consumption
Rough idling	Reduced engine power

Troubleshooting Common Problems

1. Engine Runs Rich

Symptoms: Black smoke from exhaust, poor fuel economy, hesitation during acceleration. Causes: Too much fuel entering the carburetor, clogged air filter, or dirty carburetor passages. Solutions: Adjust the air-fuel mixture screw, clean or replace the air filter, or rebuild the carburetor.

2. Engine Runs Lean

Symptoms: White or pale smoke from exhaust, popping or backfiring noises, difficulty starting. Causes: Not enough fuel entering the carburetor, dirty fuel lines, or clogged fuel filter. Solutions: Adjust the air-fuel mixture screw, clean the fuel lines, or replace the fuel filter.

3. Engine Stalls at Idle

Symptoms: Engine shuts off when not under load. Causes: Incorrect idle speed adjustment, dirty or clogged idle circuit, or vacuum leak. Solutions: Adjust the idle speed, clean the idle circuit, or inspect for vacuum leaks.

4. Engine Hesitates During Acceleration

Symptoms: Engine bogs down or stumbles when accelerating quickly. Causes: Too lean mixture during acceleration, dirty accelerator pump, or clogged fuel passage. Solutions: Adjust the air-fuel mixture screw, rebuild or replace the accelerator pump, or clean the fuel passage.

5. Engine Surges or Hunts

Symptoms: Engine rpm oscillates up and down while idling or cruising. Causes: Vacuum leak, dirty or malfunctioning throttle position sensor, or incorrect ignition timing. Solutions: Inspect for vacuum leaks, replace the throttle position sensor, or adjust the ignition timing.

6. Fuel Leaking from Carburetor

Symptoms: Gas dripping or pooling around the carburetor. Causes: Worn or damaged gasket, loose or cracked fuel line, or overflowing float bowl. Solutions: Replace the gasket, tighten the fuel line, or adjust the float height.

7. Carburetor Overflows

Symptoms: Gas overflowing from the carburetor into the engine. Causes: Stuck or damaged float, clogged fuel inlet needle, or incorrect fuel pressure. Solutions: Inspect the float for damage or binding, replace the fuel inlet needle, or adjust the fuel pressure.

8. Engine Backfires

Symptoms: Loud popping or banging noises through the exhaust. Causes: Incorrect ignition timing, lean air-fuel mixture, or exhaust leaks. Solutions: Adjust the ignition timing, adjust the air-fuel mixture, or inspect for exhaust leaks.

9. Engine Does Not Start

Symptoms: Engine cranks but does not fire. Causes: No fuel or spark, faulty ignition system, or clogged carburetor. Solutions: Check for fuel and spark, diagnose the ignition system, or rebuild the carburetor.

10. Carburetor Floats

– Float is damaged or worn out.
– Float is not properly adjusted.
– Float valve is stuck or damaged.
– Fuel inlet needle is worn or damaged.
– Fuel pressure is too high.
– Carburetor bowl is overflowing.