Ideal Fuel Pressure For Rochester Two-Barrel Carburetor Performance

Rochester 2-barrel carburetors have an adjustable float, a replaceable main metering jet, an accelerator pump, and an adjustable idler metering screw for each barrel. They also have an adjustable electric choke control spring, a vacuum choke pull-off, and a throttle idle control screw. The fuel pressure for a Rochester 2-barrel carburetor should be around 4-5 lbs. Flooding can occur if the fuel pressure is too high, or if there is a blockage caused by dirt or debris. To adjust the fuel pressure, you will need to refer to your motor's manual for the correct setting.

Troubleshooting a Rochester 2-barrel carburetor

Flooding

Flooding occurs when there is too much gas in the float bowl, causing gas to come over the top of the carburetor. To troubleshoot flooding, follow these steps:

• Test the needle and seat. Ensure there is no damage and that they are clean.
• Check the fuel pump pressure. Refer to your motor's manual for the correct setting, which should be around 4-4.5 lbs.
• Inspect the needle viton tip for any damage caused by excessive pressure when adjusting the float. Clean it with mineral spirits if necessary.
• Ensure the seat does not have two gaskets and that the gaskets are not damaged. Also, check if the float bowl is cracked around the seat.
• Hold down the needle gently into the seat and try blowing a small amount of air into the inlet to test for leaks.
• Inspect the power valve. Ensure it has only one gasket, and that the gasket is not damaged or cracked.
• Move the float up and down to ensure it is not hanging or rubbing against the side of the bowl or bowl gasket.
• Check if the float pulls the needle straight without binding it. Most 2-jets use a clip for this.
• Check for dirt in the needle, as this can come up from the gas tank despite the filter.

Percolation

Percolation occurs when the gas boils out of the carburetor due to a lower boiling point. To troubleshoot percolation:

• Feel the carburetor after running the vehicle to see if it is hot to the touch, indicating percolation.
• Ensure the fuel lines are away from the exhaust manifold to reduce heat.
• Add Ethanol Defender to your gas to address the ethanol problem.
• Keep all passages around the venturi and on top of the carburetor clear.
• Ensure any vents on top of the carburetor are clear to prevent a siphoning action.
• Check for gas dribbling down the carburetor throat after turning off the engine, which may indicate issues with the main discharge or venturi gasket.
• Inspect for leaks by removing the carburetor, filling it with gas, and letting it sit overnight on a piece of paper.
• If the car has been sitting for a long time, the gas may have varnished the inside of the carburetor, including the needle and seat.
• Disconnect the fuel line after stopping the engine and

Adjusting the float

• Understanding the Float System: The float system regulates fuel flow into the float bowl. Fuel enters through the fuel inlet screen or filter, passing the float needle seat and the float needle into the bowl. The fuel level rises, lifting the float pontoon, which then presses the float needle against the needle seat to shut off fuel flow. As fuel is used during engine operation, the float drops, allowing more fuel to enter and maintain a consistent level.
• Float Level Specifications: The correct float level depends on the specific carburetor number and application. There is no one-size-fits-all measurement. For plastic floats, measure the distance from the bowl gasket to the lip of the float's free end. For brass floats, measure from the gasket to the sharp edge of the float seam at the free end.
• Adjusting the Float Height: Ensure the float hangs freely and measure from the gasket surface to the lip of the plastic float or the bottom of the metal float. This step ensures the needle doesn't fall out or get stuck to the side.
• Checking Float Drop: It's crucial to ensure the float doesn't drop too low, as this can lead to flooding. A failing fuel pump or excessive float drop can cause the needle valve to jam and result in a flooded engine.
• Testing and Adjusting Fuel Level: With the engine cold and the ignition key off, observe the choke plate as you open the throttle by hand. The choke should snap closed when the engine is cold. If it doesn't, adjust the choke spring housing until the choke closes completely. If the choke closes when cold but doesn't open fully after the engine warms up, adjust the housing until the choke just opens all the way.
• Vacuum Choke Pull-Off: Ensure the vacuum pull-off pulls the choke open slightly as the engine starts. If it doesn't, check the vacuum hose for leaks. If there are no leaks and there is vacuum to the pull-open, replace it.
• Setting the Correct Idle Speed: Start the engine and let it warm up to normal operating temperature. Adjust the screw on the throttle lever to achieve an idle speed of 800 rpm.
• Adjusting the Idle Mixture: Use the two idle mixture screws located at the front bottom base of the carburetor. Turn the left idle mixture screw slowly while monitoring the tachometer. As the RPM starts to drop, stop and turn the screw out until the RPM decreases again. Repeat the process on both sides to fine-tune the idle mixture.
• Preventing Flooding: Flooding occurs when there's too much gas in the float bowl. To prevent this, ensure the needle and seat are functioning correctly, and check the fuel pump pressure, which should be around 4-4.5 lbs. Inspect the needle viton tip for damage and clean it if necessary.
• Final Checks: Ensure the float moves freely and doesn't hang up or rub against the bowl or gasket. Check the float clip that connects the float to the needle, ensuring it pulls the needle straight without binding.

Checking fuel pump pressure

Step 1: Understanding the Process

Before you begin, it's important to know that the fuel pump's primary role is to deliver fuel from the tank to the carburetor. Any issues with the fuel pump can lead to fuel starvation, which will cause the engine to run erratically or not start at all.

Step 2: Initial Checks

If your car suddenly stops running and you know you haven't run out of fuel, the first thing to check is whether there is fuel in the carburetor. Disconnect the fuel line to the carburetor, being careful to catch any fuel with a plastic bag secured with a rubber band. Then, turn the engine over with the starter. If fuel spurts out, the issue lies with the carburetor. If no fuel or only a small amount emerges, you likely have a blockage or leak in the fuel line or a faulty fuel pump.

Step 3: Testing Fuel Pump Delivery

To specifically test the fuel pump's delivery, you can perform the following steps:

• Disconnect the fuel line at the carburetor.
• Crank the engine.
• Observe if the pump delivers fuel as expected.
• If it doesn't pump fuel, the fuel pump likely needs to be replaced.

Step 4: Checking for Restrictions

It's important to ensure there are no restrictions in the fuel pump that could impede fuel flow. You should be able to suck fuel through the pump without much difficulty. If there is resistance or no fuel coming through, there might be an issue with the diaphragm or other internal components.

Step 5: Verifying Fuel Availability

Before condemning the fuel pump, it's crucial to rule out other potential causes. Check all fuel lines for leaks or ruptures, as these can cause the pump to draw air instead of fuel. Additionally, ensure the fuel filter and pickup tube in the tank are not clogged or restricted.

Step 6: Bench Testing the Fuel Pump

For a more comprehensive test, you can perform a bench test on the fuel pump:

• Secure the pump to a workbench or vice.
• Measure the pressure on the outlet and the vacuum on the inlet.
• Manually operate the pump lever to observe its performance.
• If the pump can push but not pull, it indicates an issue with the diaphragm or internal components.

Step 7: Checking Pump Installation

If the fuel pump is newly installed, ensure that the pump arm has engaged properly with the eccentric (fuel pump lobe). An incorrect installation can lead to pump malfunction.

Step 8: Monitoring Vacuum Levels

To further diagnose the issue, you can install a vacuum gauge on the inlet of the fuel pump. If it pulls a vacuum, the issue lies elsewhere, such as a leak in the suction lines, a faulty transfer valve, or empty fuel tanks. However, if there is no vacuum, it confirms a problem with the pump installation, pump malfunction, or an engine without the eccentric.

Step 9: Referencing Fuel Pressure Specifications

For the Rochester two-barrel carburetor, the recommended fuel pressure is approximately 4 to 4.5 lbs. However, always refer to your motor's manual for the exact specifications.

Step 10: Seeking Professional Assistance

If you are unsure about any part of the process or unable to diagnose the issue, it is best to consult a qualified mechanic. They will have the expertise and tools to accurately identify and rectify any problems with your fuel pump or carburetor.

Cleaning the carburetor

Cleaning the Rochester Carburetor

Step 1: Disassembling the Carburetor

Before cleaning your Rochester carburetor, it is important to disassemble it carefully. Start by removing the throttle base, float bowl, and choke bowl. You may also need to take out the internal parts, including the power jet. It is recommended to refer to a repair manual or seek assistance from a professional if you are unsure about the disassembly process.

Step 2: Choosing a Cleaning Method

There are several methods for cleaning a Rochester carburetor. One option is to use a carburetor cleaner, which can be purchased from automotive supply stores or online. These cleaners typically come in spray cans or immersion containers. Alternatively, some people suggest using diesel oil, choke cleaners, or brake cleaners for carburetor soaking or spraying. Always follow the manufacturer's instructions and take the necessary safety precautions when using any cleaning products.

Step 3: Cleaning the Parts

Once you have selected your cleaning method, it's time to clean the carburetor parts. Use a stiff nylon brush to remove heavy dirt and grime from the parts. Pay special attention to the small holes and passages in the carburetor, as these areas can become clogged. You can also blow out the passages with compressed air to ensure they are clear. If there is varnish build-up, soak the carburetor in a solution of Simple Green for about 2 hours, then rinse with hot water and blow out the passages again.

Step 4: Reassembling and Adjusting the Carburetor

After cleaning and drying all the parts, it's time to reassemble the carburetor. Refer to a repair manual or seek professional guidance if needed. Once reassembled, you may need to adjust the carburetor for optimal performance. This includes adjusting the idle fuel mixture screw and the idle speed screw, as well as fine-tuning the float level and other settings. Always refer to the vehicle's service manual or seek expert advice for specific adjustment instructions.

Step 5: Testing and Final Checks

Finally, install the cleaned and adjusted carburetor back into your vehicle. Start the engine and let it warm up. Test the carburetor by paying attention to any unusual noises, smells, or performance issues. Ensure that there are no leaks, and all vacuum hoses are properly connected. If everything functions correctly, you have successfully cleaned and adjusted your Rochester carburetor.

Adjusting the choke

Before beginning any adjustments, ensure that your engine is in good running condition, with all cylinders functioning properly and the timing adjusted to the recommended idle speed. It is also important to verify that there are no vacuum leaks at the intake manifold. Start by removing the air cleaner and ensuring that the engine is cold, with the ignition key turned off.

Observe the top choke plate as you slowly open the throttle by hand. The choke should immediately snap closed when the engine is cold and the throttle is activated. This mechanism is controlled by a spring located within the black circular housing on the carburetor. When the engine is cold, the spring contracts, exerting pressure on the choke to close it. However, when the carburetor is left as it was when the engine was shut off, the choke cannot close with the throttle in the closed position. As soon as the throttle is opened, a tab on the throttle enables the choke to close completely, increasing the vacuum to draw fuel into the engine for ignition.

If the choke fails to close entirely when the engine is cold and the throttle is moved, you will need to adjust it. To do this, loosen the screws on the choke spring housing and rotate it until the choke is fully closed. On the other hand, if the choke closes completely when the engine is cold but fails to open fully after the engine has started and warmed up, you will also need to make an adjustment. In this case, adjust the housing until the choke just barely opens. If the choke still lacks the tension to close, even after adjustments, it is likely that the choke spring needs to be replaced.

The vacuum choke pull-off is designed to slightly open the choke as soon as the engine starts, allowing just enough air in to keep the engine running. Once the engine is running, the electric choke spring begins to heat up and expand, gradually opening the choke until it is fully open. Check the vacuum pull-off to ensure that it pulls the choke open a small amount when the engine starts. If it does not, inspect the vacuum hose for any leaks. If there are no leaks and the vacuum is functioning correctly, you may need to replace the pull-open.

Finally, start the engine and allow it to warm up to its normal operating temperature. Use a tachometer to check the idle speed, which should be around 800 rpm. If the idle speed is not within this range, adjust the screw on the throttle lever until it reaches the desired rpm.

In summary, adjusting the choke on a Rochester 2-barrel carburetor involves ensuring the choke closes and opens correctly, fine-tuning the choke spring housing, checking the vacuum pull-off, and setting the correct idle speed. By following these steps, you can optimise the performance of your carburetor and maintain a well-running engine.

Frequently asked questions