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Carburetor Problems And Solutions

August 24, 2022  -  Fuel, Carbs, & Intakes

8 People Found This Article Helpful
holley street avenger carburetor
Holley Carburetor Street Avenger Electric Choke Vacuum Secondary 670 CFM
edelbrock carburetor
Edelbrock Carburetor Electric Choke 600 CFM
demon carburetor
Demon Carburetor Street 625 CFM

Carburetor problems and solutions for them are common in any application and simple to take care of with the help of some research. Often, if the carburetor is the confirmed component causing performance issues, a rebuild kit or simple adjustment will resolve it and bring it back to perfect working order. Carburetor issues can sometimes be elusive to determine, but with some careful inspection and basic understanding, you can fix them with ease. In this article, we will cover some commonly experienced performance issues and more to help you understand how to repair them and get your vehicle out of the garage and back on the road.

How To Know If You Have Carburetor Problems

There are many things that can cause carburetor problems — and as many ways to know how to tell if there is a carburetor issue with your car. If the vehicle engine stalls when stopping, for example, there may be an idle circuit, fuel pressure, or idle speed issue that needs to be confirmed and resolved. However, if your engine is having performance issues, the first thing to determine is if the problem is fuel-related or spark-related. This includes ensuring ignition timing is correct and has not changed. Once you identify that the engine has the proper spark to each cylinder, you can move on to the fuel system.  When checking the fuel system, start with fuel supply and pressure. Confirm the age of the fuel filter, and replace it if necessary. Next, with a fuel pressure gauge, confirm the engine is getting between 4-6 psi while running. If the pressure is less or more than the acceptable range, repair or replacement is needed to move forward with testing. Once fuel pressure is confirmed to be in the correct range, the next step is carburetor removal, disassembly, and reassembly. 

fuel pressure gauge
Confirm the Fuel Pressure with a Gauge
fuel filter
Replace the fuel filter if you are not sure how old it is

How To Fix Carburetor Problems

Once the ignition and fuel supply/pressure have been confirmed to be working, move on to the outside of the carburetor. Problems might be noticeable through signs of leaking, damage, or missing components from an external inspection. Confirm the choke (if it has one) is operating properly when cold and warm per the manufacturer's specifications to rule out carburetor choke problems. When the engine is at operating temperature, the choke plate should be fully open. When the engine is cold, the choke plate should be almost closed with an air gap matching the manufacturer's specification. 

If the engine runs properly and no changes were made to it prior to the engine performance issue, the next step is taking the carburetor apart for inspection. A carburetor rebuild kit is recommended for this step as the original gaskets and seals likely cannot be reused. Any corrosion or contamination needs to be cleaned out with a soft bristle brush and carburetor cleaner. Confirm there are no clogged jets, channels, or passageways. Make sure the float is properly set both when hanging and when upside down (for Edelbrock and other carburetors) to the manufacturer's specifications and adjust if needed. Use both carburetor cleaner and compressed air to blow out any dirt or plugged passageways. If any components look worn or damaged, replace them. 

Once everything internally is clean and confirmed properly adjusted and installed, reassembly of the carburetor can take place. Once reassembled, confirm all of the external fuel, electrical (choke), linkage, and other connections are in place. Also, make sure the carburetor-to-intake gasket is sealed correctly. Then, start the engine, recheck that the idle is between 800-1000 RPMs, and adjust if needed. Also, check the fast idle setting and match it to the manufacturer's specification. If a rebuild didn't solve the carburetor issues, replacing it with a different carburetor may be an option. However, if a new carburetor does not solve the issue, the problem may not be the carburetor but something else, including internal engine operation.  

electric choke carburetor
Electric chokes usually include a black round housing
manual choke carburetor
Manual chokes have a connection for a choke cable on a lever that controls opening and closing

Additional Questions About Carburetor Issues

Below are some additional questions people often have related to selection as well as carburetor problems and our solutions / answers to them.

How Do I Select The Correct CFM Carburetor For My Application?

The carburetor CFM is a very critical choice in your build — the wrong size can lead to significant carburetor problems. Too large will cause low RPM issues and a rich condition. Too small will restrict the engine performance at high RPM. Our techs commonly use a simple formula to select the proper CFM:

(Cubic inch of engine X Max RPM) / 3456 = maximum CFM required

Ex. (350 ci X 6500 RPM)/3456=658.28 CFM.  A 650 CFM carburetor would be ideal for this application. If the answer is between carburetor sizes, it is recommended to go with the smaller size carburetor if you are primarily using the vehicle on the street and the larger size carburetor if the vehicle will be used mostly at the track.  

Should I Use A Vacuum Secondary Or Mechanical Secondary Carburetor?

For street cars, the vacuum secondary carburetor works best with an automatic transmission. They are more forgiving than a mechanical secondary because they work by sensing engine load. To avoid carburetor issues, the mechanical secondary carb is best on a lighter car with a radical camshaft, a lower gear, and manual transmission. A mechanical secondary is also the best choice for most high RPM race applications.

holley vacuum secondary carb
Vacuum secondary housing on a Holley carburetor connected to the baseplate linkage.

I Have A 650 CFM Carb That I Want To Jet To A 750. How Can I Do This?

Actually, you can't. CFM (cubic feet per minute) refers to the air flow through the main venturis of the carburetor. Simple re-jetting will not change the carburetor CFM. Instead it will cause your engine to run either too lean or too rich, which will cause engine performance issues and even damage in some cases.

I Have A Problem Seeing Or Smelling Fuel In My Oil; Why?

This is one of the more common carburetor problems people face. When too much fuel enters the combustion chamber and engine cylinders, it can make its way past the pistons and down into the oil pan. Fuel in the oil can be caused by several conditions, including a ruptured fuel pump diaphragm, incorrect fuel line hook-up, or carburetor flooding. If the problem is carb flooding, check the following: 

  • Fuel pressure should not exceed 6.5 psi.
  • Verify the float level.
  • Verify that the needle & seats are free of any debris and have a smooth range of operation and travel.
  • Verify that the carburetor float is free of any fluid by shaking it or submerging it in a safe, non-flammable fluid.

Lastly, a fuel smell in oil can be caused by an excessively rich tuned carburetor.

brass carb float
Carburetor Float
carburetor needle and seat
Needle and Seat

I Purchased The Correct Carburetor For My Application, Installed It, And It Does Not Run Right. What Should I Do To Fix This Problem?

If you're having carburetor issues immediately after installing, the most common answer is that it needs to be tuned to match your engine. Rarely will a new carburetor work right out of the box — some tuning will be required that may include different size carburetor components such as jets, power valves, metering rods, or step-up springs. The adjustment method can differ with the carburetor type, but the general progression is the same. Use the following guide for direction, and refer to the carburetor owner's manual for specific methods.

vacuum fuel pressure gauge tester
Vacuum / Fuel pressure gauge
  • First, you will need to be sure the engine is warm and in good working order with correct ignition timing.
  • Be sure that the float level and fuel pressure are within the specified ranges.
  • The first circuit to tune is the idle circuit; this requires the use of a vacuum gauge to adjust the idle mixture screws, tuning to maximum vacuum.
  • The second circuit to tune is the intermediate circuit, the most common circuit that needs to be tuned. This circuit is most responsible for off-idle hesitations or stumbles. As mentioned, each carburetor is different, but this circuit typically involves a mechanical lever that compresses an accelerator pump diaphragm through a discharge nozzle. In almost all types of carburetors, the mechanical pump action is adjustable and so is the orifice size of which the fuel is dispersed.
  • Next is secondary jetting/tuning. On most vacuum secondary carburetors, the rate at which the secondaries open is adjustable. It may be advantageous to open the secondaries earlier or later. Road testing and trial and error is the most common way to come to this determination.
  • Lastly, strict jetting is fairly straight forward. If the engine runs lean at wide open throttle, it will need to be jetted up; if rich, it will need to be jetted down. Spark plug reading is a very common method to determine WOT(wide open throttle) tuning; some more advanced tuners will use an air/fuel ratio gauge that reads fuel mixture from the exhaust pipe.

I Can't Get My Street Car To Idle Below 1,000 RPM; How Can I Fix This?

A high idle is an indication that more air is getting into the intake manifold than necessary. You may be experiencing carburetor choke problems — check for correct choke operation and adjustment. Make sure that the fast idle cam is not causing this. Verify that the throttle arm rests on the idle speed screw. It is important to verify throttle linkage and that throttle blades are not binding — they should have a free range of operation. The throttle return spring should be correctly located and adjusted. In most cases, the return spring should be positioned on the top of the throttle arm forward. Make sure all of the vacuum ports on the carburetor are being utilized or blocked off. Check for any vacuum leaks, when the idle mixture screw(s) are all adjusted inward, the engine should die. If the engine remains running, this is a key indication of a vacuum leak.

How Do I Hook Up My Electric Choke?

Typically, an electric choke will involve 2 wires. The first and most critical is the, often red, positive voltage wire. This is to be connected to a switched 12-volt ignition source that provides power only when the ignition key is on. Connecting this wire to a full-time 12-volt wire will cause damage to the choke and drain the battery when the key is out of the ignition. The second wire is the, often black, ground wire. Simply connect this to a chassis ground. Most manufacturers pre-terminate this wire to the carburetor base or body. This is ok as long as your engine ground is present and functioning.

electric choke cap
Electric choke carburetors only need two connections: ground and 12-volt switched power.

Where Is the Proper Vacuum Port To Hook My Distributor Vacuum Advance?

It is critical for your distributor vacuum advance to be hooked to a timed port. If this is connected to a full-time vacuum port, it will remain advanced full time and you will lose the functionality of the vacuum advance feature. The timed port on a Holley-designed carburetor is located high on the side of the primary metering block. On the Carter AFB/Edelbrock design, it is the higher of the two small vacuum ports on the front of the carburetor. Quadrajets can differ with design, but their timed port is typically placed mid level on the rear right side. In all cases, the correct vacuum port will be above the throttle blades of the carburetor.

[image: edelbrock vacuum fuel connection diagram] [image: holley carburetor vacuum fuel connection diagram]

edelbrock fuel vacuum connection diagram
Edelbrock carburetors connect to distributor with left side vacuum port
holley fuel vacuum connection diagram
Holley carburetors connect to the distributor with a port on the metering block. 

Why Does My Engine Stall/Hesitate When I Hard Stop, Hard Accelerate, Or Turn A Sharp Corner?

If your engine stalls when stopping, accelerating or turning, this is likely a float level issue. Confirm the float level and retest the cause of the stall. In racing applications, specialized floats are available for extreme versions of these situations. Also, the use of jet extensions may be required in the secondary side of a carburetor for extremely hard accelerations, such as a drag car. Another area to check is fuel supply and pressure. Confirm the fuel pressure is between 4-6 PSI. If the fuel pump is not supplying enough fuel to the carburetor, under hard acceleration the fuel bowls may empty, causing flat spots, or stalls as the pump struggles to supply more fuel than the engine uses. 

mechanical fuel pump
Mechanical Fuel Pump
electric fuel pump
Electric Fuel Pump

How Can I Lean Down My Carburetor To Get Better Mileage?

First off, to avoid carburetor problems, make sure you have the correct CFM for your application and the idle mixture screws are properly adjusted. You can make your carburetor leaner, but this will not always improve gas mileage. Since every engine is different, there is no way of knowing how much of an effect leaning down the carburetor will have on your engine. You can refer to your carburetor owner's manual and the calibration suggestions inside to lean down the carb. Start off light, but keep an eye on the plugs to make sure you aren't taking it too lean. The most efficient fuel mixture typically results in a light to medium tan color on the internal porcelain area of the spark plug. White or gray porcelain indicates a lean condition; black or fuel-soaked porcelain indicates a rich condition. Use great care as going to lean will cause engine damage. 

Solve Your Carburetor Issues With Parts From JEGS

If you're experiencing carburetor problems and solutions require new parts or a full replacement, JEGS has you covered. We offer a great selection of carburetors to meet your engine's needs, along with a full selection of carb options, including an easy-to-install JEGS street carburetor. If you need help, feel free to contact JEGS today.

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