Otis Singleton
The Quadrajet carburettor is a popular choice for car enthusiasts, but setting up the electric choke correctly can be a challenge. The choke should only be powered when the key is turned to Run, and not in any other position, including Crank. It is important to prevent the engine from flooding by adjusting the fast idle, and getting the adjustment right requires following the instructions that come with the carburettor kit or manual. With a cold engine, the choke should be set to just closed, and as the engine warms up, the butterfly should start to open.
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What You'll Learn
Adjusting the choke pull off
Adjusting the choke pull-off on a Quadrajet carburettor is a crucial step in ensuring optimal engine performance, particularly during cold starts. Here is a detailed guide on how to adjust the choke pull-off:
Firstly, it is important to understand the components involved. The choke pull-off is a vacuum solenoid that controls the opening of the choke blades. It is often identified by its white plastic diaphragm. The choke plate or choke blade is the component that needs to be adjusted for the correct opening.
Now, for the adjustment process:
- Start with a cold engine, preferably one that has been sitting overnight. This is crucial as choke adjustments are sensitive to engine temperature.
- Close the choke plate by hitting the throttle.
- Apply vacuum to the choke pull-off. This can be done using a vacuum pump or by sucking on a vacuum hose attached to the choke pull-off.
- Adjust the screw on the choke pull-off arm until there is a precise gap of 1/8 inch between the choke plate and the carburettor body. This gap ensures the choke is not too rich, preventing loading up.
- If the choke plate is not shutting completely, you may need to bend the rod coming up from the intake until it closes fully, being careful not to over-tighten it.
- Fine-tune the adjustment by starting the engine and observing the choke blade's behaviour. Adjust the choke pull-off screw as needed to ensure the choke opens about 1/8 inch at the top as soon as the engine starts.
- Pay attention to the fast idle cam. When the choke pull-off opens, tap the gas, and ensure the fast idle cam drops to the second step.
- Test the engine's response during cold starts. If the choke pull-off is adjusted correctly, the engine should start with ease.
Remember, choke adjustments can be finicky, and you may need to make seasonal adjustments if you experience significant temperature variations. Always refer to the carburettor's instruction manual or a reputable repair guide for specific details pertaining to your Quadrajet carburettor.
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Wiring the electric choke
Wiring an electric choke on a quadrajet carburettor can be a tricky process. Firstly, it is important to note that the electric choke should only have power when the key is in the "RUN" position and not in any other position, including "CRANK". This is to ensure that the choke does not open before the engine starts, which can lead to cold starts and a lean mixture. One way to achieve this is to route the power through an oil pressure switch, so the choke loses power if the engine stalls. Alternatively, you can wire the electric choke to a switched 12-volt source that is on when the ignition is on. The fuse box is a good place for this.
When adjusting the electric choke, it is important to prevent the engine from flooding by adjusting the fast idle. This can be a delicate balancing act and may require fine-tuning. One method is to have someone start the engine while you keep your fingers on the choke blade to feel the starting position and make adjustments as needed. Additionally, some carburettors have a secondary choke pull-off on the passenger side rear, which can cause hot start and fast idle issues. This secondary choke is vacuum-controlled by the thermal vacuum switch in the air cleaner.
To adjust the choke blade position, you can loosen the three screws, turn the choke assembly to the desired position, and then retighten the screws. The choke blade should be just closed when the engine is cold, and as the engine warms up, the butterfly should start to open. This can be adjusted with the Phillips head screw on the choke pull-off arm.
Additionally, an in-line resistor can be used to lower the current to the choke coil, extending the time it takes to open. This can be a useful modification for cold starts.
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Balancing the fast idle
Firstly, it is important to note that the choke should be fully open and the engine should be warm. With the engine off, locate the base idle set speed screw on the left side of the carburettor (driver's side) and the two fuel mixture screws at the front of the base plate. You will also need to locate the fast idle set speed screw on the right side (passenger side) by the choke.
Now, adjust the idle screw to the desired rpm, which is typically 650 rpm for automatic transmission and 750 rpm for standard transmission. Once the idle speed is set, turn the left idle mixture screw out until the highest rpm is achieved. Then, turn it back in by a quarter turn for lean idle adjustment. Adjust the idle back to the desired rpm.
Next, adjust the right idle mixture screw in the same way. Turn it out until the highest rpm is reached, then turn it back in by a quarter turn. Reset the idle adjustment screw to the desired rpm. This process should be done in small increments to maintain the balance of the idle circuit.
If you are still experiencing issues with the fast idle, there are a few other adjustments you can make. One is to adjust the choke cover by rotating it clockwise to shorten the amount of time the choke is on. You can also adjust the screw at the end of the choke pull-off linkage, which controls how rich or lean the choke runs. Finally, ensure that the choke fully opens when the engine is warm and that the pull-off opens the choke to around 1/4" to 5/16" when the engine fires.
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Setting the choke blade
Firstly, the choke blade should be set to the desired position. This can be done by loosening the screws and turning the choke assembly, and then retightening the screws. The desired position will depend on the climate and the engine temperature. For example, in colder climates or during winter, you may want to leave a small gap of around 1/8" when the butterfly is closed.
The choke pull-off adjustment can be fine-tuned by having someone start the engine while you have your fingers on the choke blade to feel the position it starts at. This will allow you to make any necessary adjustments.
Additionally, the choke thermostat can be adjusted to bring the choke blade to the closed position when the engine is cold. As the engine warms up, the choke blade should start to open gradually.
It is also important to note that some carburetors have a secondary choke pull-off that is vacuum-controlled. This can cause issues with hot starts and fast idle, so adjustments may be needed to balance these factors.
Finally, an in-line resistor can be used to lower the current to the choke coil, which will extend the time it takes to open. This may be a solution to consider if you are having issues with the choke stalling or not opening quickly enough.
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Converting to a hot air choke
The choke should only have power in the "run" position. You don't want to turn the key to "ACC" and have it open before you start the engine. You can wire it to a switched 12-volt source that is on when the ignition is on. The fuse box is a good place for this.
When adjusting the choke pull-off, some users like to have someone else start the engine while they keep their fingers on the choke blade to fine-tune the choke pull-off adjustment. You can also adjust the choke pull-off by the Phillips head screw on the choke pull-off arm.
Some carbs have a secondary choke pull-off on the passenger side rear of the carb, which can cause all kinds of hot start issues and fast idle issues. The secondary choke pull-off is vacuum-controlled by the thermal vacuum switch in the air cleaner. When the air in the air cleaner reaches a specific temperature, usually around 105 degrees Fahrenheit, the secondary choke pull-off is yanked open.
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Frequently asked questions
With the engine cold, set the choke to lightly close and fire the engine. If it pulls off too slowly, it is set too tight. You can also try having someone start the engine while you have your fingers on the choke blade to fine-tune the choke pull-off adjustment.
Some sources recommend leaving a small 1/8" gap when starting.
You should only have power in the "run" position. You can wire it to a switched 12V source that is on when the ignition is on.
