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CARBURETOR SERVICE

Tool and Material Checklist:

  • Air filter 
  • Aerosol carburetor cleaner
  • Gas filter 
  • Choke solvent
  • Fuel system cleaner additive 
  • Tape, pencils, or golf tees
  • Service manual
  • Gravity-feed carburetor cleaner kit
  • Screwdriver  
  • Choke coil
  • Rag or can

The carburetor meters, atomizes, and distributes the fuel throughout the air flowing into the engine. It also allows the driver to regulate the amount of air/fuel mixture that enters the engine, thereby controlling engine speed. This booklet explains how to keep the carburetor functioning properly.

REPLACING FILTERS

The best way to keep your carburetor clean and working efficiently is to replace the air and gas filters regularly (at least once a year). When this is done, the carburetor will frequently last the life of the engine without major maintenance.

CLEANING THE CARBURETOR

Dirt and other contaminants inside the engine are major contributors to engine wear. Most dirt enters the system through the carburetor, where it causes innumerable problems, including hard starting, lagging acceleration, stalling, excess emissions, and poor gas mileage. The importance of keeping the carburetor clean is obvious, and there are several ways this can be done.

Cleaner Additive

The easiest method of carburetor maintenance is to add a fuel system cleaner to the gasoline during every oil change. It does a fairly good job of keeping the carburetor clean, especially if it is used from the time the car is new. However, it is not very good at removing built-up dirt.

Cleaner Kit

The most effective method of cleaning a carburetor. short of removing it from the engine, is to run solvent directly into it using a special gravity feed kit. The procedure is as follows:

1. With the engine off, remove the air cleaner assembly.

2. Disconnect the gas line at the carburetor. Catch the escaping gasoline in a can.

3. Block off the gas flow with the special block off fitting provided in the kit.

4. Attach the flexible solvent hose to the gas inlet of the carburetor, using the correct size adapter from the kit.

5. Install the other end of the hose to the can of solvent and suspend it from the underside of the hood.

6. Start the engine. Run it at varying speeds so the solvent penetrates through all of the jets and passages.

7. Periodically place your hand over the carburetor to block the airflow for a brief period of time. This will force the solvent through the unit.

8. The engine will die when all of the solvent has run through the carburetor. The internal passages should be fairly well cleaned by this time.

9. Carefully reconnect the fuel line.

10. Start the engine, then spray a can of aerosol carburetor cleaner/de-carbonizer down the throat of the carburetor. Be sure to follow the instructions on the can.

11. Clean the outside surfaces of the carburetor (especially the linkages) with choke solvent.

12. Check for any gasoline leaks originating at the carburetor or fuel line.

REBUILDING THE CARBURETOR

If the original problem, such as stalling or poor gas mileage, still exists after cleaning, the carburetor might have to be rebuilt or replaced. However, since other types of performance troubles can easily be mistaken for a fuel system problem, a complete diagnosis should be made before considering a carburetor rebuild.

ADJUSTING THE FLOAT LEVEL

Fuel in the carburetor float bowl must be maintained at a specific level to achieve correct fuel metering under all driving conditions. If the float level is too high, the air/fuel mixture is too rich; the result is flooding, hard starting when hot, rough idle, stalling at turns and stops, excess emissions, and poor gas mileage. If the float level is too low, the air/fuel mixture is too lean. This condition is indicated by hard starting when cold, coughing and spitting, and acceleration delay.

The float system is perhaps the single most important system in the carburetor, since the correct operation of all other systems depends on the proper level of fuel in the float bowl. Use the following procedure to adjust the float level:

1. Remove the air cleaner housing.

NOTE: On some cars, the float level can be adjusted from outside the carburetor while the engine is running; removing the air cleaner provides access to the external adjustment.

2. Mark any disconnected hoses for identification. Use tape or pencils to plug the ends.

3. Check the service manual for the float level specification and carburetor disassembly instructions.

4. Remove the float from its arm, being careful not to lose the float needle.

5. Shake the float to see if there is fuel inside. Check for dents; a float with a leak or a dent should be replaced. 

NOTE: Shaking the float wont work If it isn't hollow inside. Solid floats must be weighed using a float scale.

6. Attach the float back on its arm.

7. Adjust the float to the designated level by bending the tab on the float arm. The tab is the projection that contacts the float needle. Do not bend the float or its arm and be careful not to twist the float.

Sometimes a float or needle can stick in the open or closed position, often due to needle seat wear. As a result, fuel cannot flow into the float bowl and the engine stalls. If the needle shows signs of wear, replace it and the seat according to the service manual instructions.

ADJUSTING THE IDLE

Idle adjustments should only be made after the float level has been adjusted. Refer to "Engine Tune-up Basics."

MAINTAINING THE CHOKE

Most engines run on an air/fuel mixture of about 14 parts of air to every 1 part of fuel. However, a cold engine can't ignite such a lean fuel mixture. To start a cold engine, more fuel and less air must be supplied to the combustion chambers. The choke closes off the carburetor throat, allowing less air to enter, while at the same time creating a very high vacuum in the venturi area of the carburetor, which sucks large amounts of fuel into the engine. With less air in the carburetor, the percentage of fuel going to the combustion chambers increases and combustion occurs more readily. As the engine warms, the choke plate gradually opens, allowing more and more air to enter the carburetor until the engine is burning the normal fuel mixture.

When a choke is not functioning, it is usually obvious; the car won't start, or if it does, it won't keep running. But frequently, a sticking or improperly adjusted choke goes unnoticed. Since either of these conditions causes excess fuel consumption and increased exhaust emissions, the choke should be checked regularly even when the engine is not experiencing starting problems.

Types of Chokes

The two most common chokes are the well and the in-carburetor types. They both use a temperature-sensitive metal coil and a special vacuum control unit to regulate the operation of the choke plate.

A well choke has the thermostatic coil located in a special well on the intake manifold and is identified by a connecting rod running from the coil to the choke linkage at the carburetor. It also uses an external vacuum control diaphragm.

On an in-carburetor choke, the thermo coil and vacuum control are actually part of the carburetor. The in-carburetor choke is identifiable by the round thermostatic coil cover located on the back or side of the carburetor.

Choke Operation

Both the well and in-carburetor chokes operate in essentially the same manner. The vacuum unit, or choke pull7off, is attached to the choke plate linkage and is responsible for opening the plate. When the engine is started cold, the thermostatic coil tension is very high and attempts to hold the choke plate closed. As the engine warms, the vacuum unit slowly overcomes the coil tension and pulls the choke plate open.

Cleaning the Choke Assembly

Dirty or sticking linkages are by far the most frequent causes of choke problems. The most important choke maintenance is a regular, thorough cleaning of the entire assembly with an aerosol choke solvent. On in-carburetor chokes, spray the internal surfaces (especially the vacuum piston and bore) as well. Use only a choke solvent or carburetor cleaner; oil will cause dirt to collect.

Inspecting the Choke

Other than sticking linkages, a choke that opens too quickly or too slowly is the most common problem. To inspect the choke, follow these steps:

1. Remove the air cleaner top.

2. Observe the choke plate. With the engine cold and not running, it should be closed.

3 If the choke plate is not closed, the choke coil needs to be adjusted or replaced. Consult the appropriate sections in this booklet.

4. Start the engine.

5. Observe the choke plate again. If it cracks open slightly, then gradually opens as the engine warms, the choke is operating correctly.

Keep in mind that the choke might need adjusting even after this check. If the plate opens too fast, the engine will stall and waste gasoline until it warms up. More frequently, the plate will open too slowly. Not only does this waste gas, but it also promotes engine wear by allowing excess raw gas to enter the engine and dilute the oil. Dark exhaust smoke during warm-up is a sign of this problem.

Adjusting an In-Carburetor Choke

1. Remove the air cleaner assembly.

2. Locate the plastic coil cover on the back or side of the carburetor.

3. Loosen the three screws securing the cover.

4. The cover is calibrated with lines showing richer and leaner settings. There is also an arrow pointing in the direction of the leaner settings.

5. Move the cover one line in the appropriate direction. A leaner choke setting reduces tension on the coil and allows the choke to open sooner. A richer choke setting holds the choke closed longer.

6. Tighten the screws and test the new setting with a cold start. Experiment with the setting to find the best one for the engine and climate. The leanest setting that will still start the engine and keep it running is usually best.

Adjusting a Well Choke

1. Check your service manual to see if the choke is adjustable; many well chokes are not. If a malfunctioning well choke is not adjustable, the entire coil assembly must be replaced.

2. If the choke is adjustable, begin by removing the coil from the well.

3. Loosen the locknut that secures the calibration plates.

4. Realign the lines for a richer or leaner setting. The standard setting is one line rich.

THERMOSTATIC COIL REPLACEMENT

Replace the thermo coil when the choke plate fails to close with the engine cold or when it tails to open correctly after adjustment.

For an in-carburetor choke, follow these steps:

1. Remove the screws securing the plastic cover.

2. Remove the coil from inside the cover.

3. Install the new coil.

4. Replace the cover, making certain that any spacers or washers are correctly installed. Be sure the cover attaches to the choke plate linkage in the same manner as the original coil.

5. Set the choke to factory specifications and try a cold start.

For a well choke, follow these steps:

1. Loosen the nuts on the well cover.

2. Remove the cover.

3. Disconnect the connecting rod at the choke linkage.

4. Lift the coil assembly from the well.

5. Install a new coil on the end of the rod.

6. Place the entire assembly back in the well.

7. Remember to reattach the connecting rod at the carburetor.

 

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