Three Reasons Classic Cars Need a Carburetor Overhaul

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4 Barrel Carburetor on a 427 Chevy. Photo by Mark Gittelman

Although it's possible for a classic automobile to come with with fuel injection, chances are, it has a carburetor instead. Often when a vintage automobile is hard to start, runs poorly, will not idle or has a severe hesitation, we trace the problem back to a malfunction inside the carburetor. Here we’ll cover the three main reasons your classic car needs this component overhauled and the symptoms associated with each.

Accelerator Pump Problems

When the driver starts to push down on the gas pedal an extra amount of fuel is needed to transition from the idle circuit to the part throttle mode of operation. Without this extra shot of gas there would be a noticeable hesitation as the throttle opened. The carburetor has an accelerator pump that handles this enrichment. It is a metal stem with a rubber cup at the bottom. It fits down into a column of reserve liquid fuel.

The top of the stem sticks out through the upper half of the carburetor and makes contact with a piece of throttle linkage. As the driver steps on the accelerator the stem travels downward and pushes the column of fuel from the reserve into the venturi of the carburetor.

Symptoms of Accelerator Pump Issues

The most common symptom associated with an accelerator pump problem is engine hesitation. The proper operation of the pump can be verified by moving the throttle and looking through the top part of the carburetor.

When functioning correctly, two powerful jets can be observed shooting down into the carburetor as the throttle is moved.

The accelerator pump can last a long time because gasoline helps to lubricate it and therefore prolongs its life. Unfortunately, if the car sits for a long time, the alcohol and water in the fuel can separate from the gas and deteriorate the delicate rubber cup of the accelerator pump.

Replacement parts are only available in a carburetor overhaul kit, because the unit must be disassembled to replace it. New gaskets will be required for reassembly. The overhaul kit also includes new springs and check balls if it is equipped with them.

The Carburetor Needle and Seat

For a carburetor to function properly, it needs a reserve amount of gasoline. This is stored in the fuel bowl. The device that measures the amount of gas in the bowl is called a float. The float in turn controls a needle valve and seat connected to the pressurized fuel system. When the float rises to the top of the bowl, it pushes down on the needle, sealing off any additional fuel from entering the bowl.

The problem with this arrangement, although it works well, is the tip of the needle is made of a rubberized material to enhance its sealing capabilities. Since gasoline is a petroleum product this rubber needle has a long life. However, since modern fuels include high amounts of alcohol and detergents these can work against the rubber needle. Deterioration is inevitable.

Symptoms of a Bad Needle and Seat

When the needle deteriorates, it loses its ability to turn off fuel entering the carburetor. The most common symptoms associated with this condition would be an engine flooding scenario.

This can cause hard starting or extended cranking periods. It is easy to confuse this type of problem with carburetor choke problems.

After the engine starts it may emit a large amount of black smoke signifying a rich condition. Another symptom of this kind of internal problem would be the engine runs better at higher RPMs and struggles to idle properly. Since fuel continues to enter the carburetor higher RPMs burns the excess fuel. When less gas is needed at idle the engine struggles to stay running and can stall.

Deteriorating Carburetor Gaskets

Although carburetor gaskets were designed to have long life the original engineers probably didn't figure they would have to last fifty years or more. The gaskets are made from a composite material resembling a thick strong paper. Unlike accelerator pump problems and issues with the needle and seat, gasket failure often occurs because the carburetor starts to loosen up.

A carburetor assembly is three separate pieces clamped together with gaskets in between.

As the screws clamping the three pieces together start to loosen, it allows raw gas to access the gasket material. This can start a chain of deterioration that eats away at the gasket. When sealing is compromised, it can cause fuel leaks, both internal and external. Internal failure is when it leaks inside the carburetor forcing a rich condition with symptoms similar to a bad needle and seat.

External fuel leakage is visible in the area around the intake manifold. Gasket deterioration can also cause vacuum leaks. This is considered a lean condition with common symptoms like an engine misfire or hesitation. Once again, these carburetor gaskets are sourced through a purchase of an overhaul kit. Accuracy is important when shopping for carburetor parts. Car owners should cross check the overhaul kit with the factory part number directly from the carburetor, against the year make and model of the automobile.