How To

Mixing It Up: Carburetors

  • Photo courtesy of Barry Grant. - 0
  • The booster is the ring that resides in the venturi of the carburetor. During main metering operation, the fuel is discharged through it. The image is from an Edelbrock carburetor. - 1
  • An OE-style four-barrel has smaller throttle plates on the primary side, and larger ones for the secondary. This creates a very strong booster signal at low speeds for good atomization and throttle response. - 2
  • A race-inspired design such as this Holley has four butterflies the same size, dual accelerator pumps (both primary and secondary sides), and a mechanically operated secondary circuit instead of via air flow velocity (vacuum). - 3
  • The Rochester Quadrajet idle speed screw and mixture adjustment. Most OE carburetors had the mixture screw(s) in the throttle body for better idle quality. - 4
  • Rochester uses a plunger-style accelerator pump, and provides three holes in the linkage to modify the amount of fuel added. - 5
  • Holley employs a diaphragm-style accelerator pump with an adjustment screw. They also feature the only external float adjustment that uses a sight plug. - 6
  • An electric choke has a 12-volt heating element, and does not sense engine heat. The lines on the case are the spring tension adjustment. Some Chryslers had an electric-assisted choke that sensed manifold temperature also. - 7
  • This Rochester Tri-Power has a hot air choke (small tube) to control the release rate of the spring tension. - 8
  • Carter and Edelbrock used an accelerator pump design similar to Rochester. - 9
  • The main well plugs are usually a source of fuel leaks on a Quadrajet. These were expoxied to cure a hard starting problem when left overnight. - 10
  • Some Holley carburetors use a four-corner idle, and have a mixture screw for each barrel. - 11
  • The Carter and Edelbrock units feature externally removable primary metering rods (kidney shaped cover). - 12
  • The choke pull-off on a Carter Thermo-Quad is adjusted by bending the linkage. - 13
  • A fast idle speed is adjusted by a screw that is independent of the curb or base idle. On some Rochester carbs, a lock-out lever (not on this model) will not allow the secondary to open if the choke spring still has tension on it. - 14
  • Quadrajets have externally removable secondary metering rods, but the air horn (top) needs to be removed to gain access to the primary rods. - 15
  • Secondary air valve tension can be adjusted easily on a Quadrajet. This is the set screw for it - 16
  • On Quadrajet’s side is the spring tension adjustment. Set the tension just at the verge of bogging–but not quite–when the throttle is slammed open. - 17
  • All OE carbs offer two vacuum ports for full time (under the throttle plates), and ported signals (the throttle needs to be opened to expose the passage) for accessories. - 18
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by Butch Thomas  More from Author

Hot Tips And Tricks For A Smooth-Running Engine

Out of all of the components on an engine, the carburetor is often the most maligned and misunderstood. It is blamed for conditions that it has no control over, and when it is at fault, the entire unit is condemned instead of the single circuit that has a problem. It is about time you enthusiasts are shown how simple it is to get the carburetor on your engine to run perfectly–all you need to do is understand how it works. To this cause, Classic Cars & Parts has prepared this primer on carburetor basics and how to address the common maladies. In addition, we will cover some particulars on each brand and how to work with them.

Carburetor 101

To burn gasoline as a fuel in an internal combustion engine, three things need to occur: the fuel must be atomized, emulsified, and vaporized. This means broken down into small particles, mixed with air, and undergo a phase change into a gaseous state, respectively. The job of the carburetor is to atomize and emulsify the gasoline. The phase change (vaporization) occurs through latent heat as a function of physics, and this happens while the fuel and air are traveling through the intake manifold. Heat is required to vaporize gasoline, and this process is identified as the latent heat of vaporization.

The level of proficiency that the carburetor achieves in atomizing and emulsifying the gasoline will, in turn, impact the efficiency of the phase change. For an engine to run properly all three steps need to occur–and since we can’t alter the laws of physics, the carburetor is the only area in question.

A carburetor functions on a principle of a pressure differential. It can be thought of as a siphon device. The atmospheric pressure is higher than the pressure in the bore of the carburetor (called a venturi), and thus, the fuel is pushed out of the reservoir (the float bowl), and discharged through a component known as a booster. A venturi is a calculated taper in the carburetor throat or bore that causes the airflow to speed up. This lowers the pressure, and creates the differential over atmosphere. Any pressure that is less than atmospheric is known as a vacuum–or more properly, a depression. The dissimilar pressure causes the fuel to flow, and though many think that gasoline is pulled from the carburetor, it is actually pushed by the higher atmospheric pressure. Remember, high pressure travels to low pressure.

The parts of the carburetor are identified as circuits. They are the float, main metering, power enrichment, accelerator pump, idle, choke, and in the case of a four-barrel, the secondary circuit. A carb needs to create the proper air/fuel ratio (the amount of fuel and air mixed together, identified as charge) for the engine operating conditions. This can vary from as rich as 4:1 on a cold start in the middle of winter, 14.7:1 while cruising down the highway, 12.7:1 when you floor the throttle to merge onto a road, or 16:1 while coasting. Thus, the different circuits need to work together with the others in a similar fashion as the human body does with its organs–each has a purpose.

A simple way of identifying the seven circuits are:

Float bowl: The fuel reservoir the carburetor uses to supply the circuits. The float keeps the level at the proper height and is constantly adjusting the volume.

Main metering: The primary fuel metering section that includes the jets and air bleeds. It is the circuit that allows the engine to run above idle speed.

Power enrichment: Supplies additional fuel when the engine needs to make the most power. It can be accomplished through a power valve (Holley) or metering rods (Rochester, Carter). The power circuit senses engine vacuum.

Accelerator pump: This circuit squirts a small stream of fuel into the venturi whenever the throttle is modulated open. It compensates for the inherent lag of the main metering system to respond to the pressure change in the venturi. It is only active while the throttle is being moved.

Idle: Allows the engine to be fueled at closed throttle (no load) when the pressure differential in the venturi is not strong enough (this is often referred to as the signal) to initiate fuel flow through the booster.

Choke: Provides the excessively rich mixture required to start the engine when cold due to the poor rate of vaporization and the low piston speed that does not allow for a strong differential.

Secondary: Additional butterflies to allow the engine to breathe more air to produce a higher level of power.

Inside a carburetor, a synergy is established and a hierarchy is created. During most engine operating stages, a building-block approach is employed that has the internal circuits working together. Due to this, a problem in a foundation circuit such as the float level will impact carb performance in every manner. Likewise, a dirty and gummed-up carburetor throat will have the air bleed holes plugged or partially blocked. The result will be performance issues since the emulsification and fuel delivery are impacted.

A basic approach

The first step in getting your carburetor to run properly is to make sure all fasteners that hold it together–along with those that attach it to the intake manifold–are tight. The carb fasteners should be made snug and not overtightened since the parts can warp. The carburetor needs to be thoroughly washed with carb spray cleaner, making sure the venturis and little air bleed holes are all varnish free. Often this is all that is required to keep it performing properly. Any diagnostics should begin with these steps, and it should be part of your normal maintenance.

Adjusting the mixture screws is a simple task that few understand. It will require the use of a tune-up style tachometer so that very small changes in engine speed can be detected. The in-car tach does not have enough resolution. Before any carburetor adjustment is to be performed, the oil in the crankcase needs to be either changed or checked for any fuel contamination by smelling it. A misadjusted choke, an excessive amount of cold starts, and short trips will all pollute the oil with gasoline. Not only does this impact the lubricant’s ability to do its job, but it will artificially richen the mixture. The gasoline fumes will be picked up by the PCV and enter the intake manifold. If you adjust the carburetor to this condition once you change the oil, the engine will be too lean and idle poorly.

Follow these steps to adjust any brand of carburetor:

1. With the engine off, remove each mixture screw, spray carb cleaner in the hole, and then reinstall it. Gently seat the mixture screw and turn it out two complete revolutions. This will balance the screws.

2. Start the engine with the tach installed. Let it warm up so that the oil is hot and the choke is fully off. Adjust the idle speed screw to the desired rpm or specification.

3. Reinstall the air filter and if the vehicle has an automatic transmission, have a helper sit inside with the wheels chocked, the emergency brake on, and the trans in drive. You want to adjust the idle mixture with the pressure drop the carburetor will see in actual use.

4. Start with one mixture screw and turn it in (lean) approximately 1/16 of a turn while watching the tach. Your goal is to achieve the highest engine speed by altering the air/fuel ratio. Be patient and wait for the engine to stabilize. When you have gone too lean, the engine speed and idle quality will drop off.

5. In approximately 1/32 increments, bring the mixture screw out (rich) until the maximum engine speed is achieved. Repeat the procedure on the other mixture screw(s). With every change, wait for the engine to stabilize.

6. Recheck the curb idle speed and, if necessary, turn the idle speed back down to the desired value.

The idle speed is very important when adjusting the mixture. If it is too high, the carburetor will start to evoke the main metering circuit and negate some of the mixture screw’s authority. For this reason the base ignition timing and idle speed adjustment need to be in harmony. If the timing is too retarded then the throttle plate(s) will need to be opened more to achieve a given idle speed. As the throttle plate is opened, it exposes a port called a transfer passage or slot. When this happens it will start fuel flow from the booster. If you look down the carburetor and see fuel dripping from a booster with the engine running, then the throttle plates are too far open, or an air bleed is plugged.

Since Detroit has not produced a new carburetor-equipped vehicle since 1988, many of the engines in use do not have their original carb–they have a rebuilt unit. The problem with many rebuilt carburetors is they possess the wrong internal parts. For example, a Rochester Quadrajet was produced for many different engines with a myriad of jet, air bleed, and metering rod sizes. All of these parts interchange, and both small and large rebuilders have been known to install different components on each side of the carburetor by mistake. I have taken apart supposed rebuilt units that have had four different jet sizes, mismatched metering rods, and the like. A carburetor like that will never adjust properly or run right. If you have an older engine with an original carb and the time comes when it needs to be serviced, do it yourself or take it to a shop that will give you yours–and not an exchange unit–back.

Common problems

Though carburetors vary slightly by manufacturer, the seven basic circuits are designed to accomplish the same task–only the approach is different. When diagnosing a driveability issue that is determined to be fuel related, a generic approach can be applied. Use the following tips to help isolate the cause of a carburetor problem.

Extended crank on first start: The float bowl is either leaking and losing fuel, or the fuel is evaporating out of it due to a stuck heat riser or similar system. The engine needs to crank excessively to refill the float bowl before it starts. To check for this, before trying to start the engine remove the air filter and move the throttle, looking for accelerator pump stroke. If the bowl is empty there will be little to no pump shot after one or two strokes. Another possible cause is the choke plate sticking open and not closing. A quick visual inspection will determine this.

Poor overall performance: This usually is indicative of the float level being too low and the engine running lean under all operating conditions.

Stalls on aggressive stop: Stalling or loading up during a panic stop can be caused by a float level that is too high.

Tip-in hesitation (stumble) when the throttle is opened: The accelerator pump is not delivering enough enrichment to compensate for the opening of the throttle plate. It is usually a weak accelerator pump plunger (Rochester and Carter) or diaphragm (Holley).

Rough idle: If no vacuum leak is present, the idle mixture needs to be adjusted. A lean mixture will induce idle instability, while the engine will be more forgiving of a rich condition.

Loads up and runs rough during warm-up: The choke pull-off or break has either failed, or is not opening the choke plate enough. It is used to open the choke slightly against spring tension to allow the engine to run. Another cause is too low of a fast-idle speed.

Balky when cold or during warm-up: The choke spring tension is too loose. The choke is coming off too soon. Tighten choke spring tension by one notch.

When to make internal changes

If your engine is stock or mildly modified (small camshaft, better cylinder heads, headers) and has the proper factory carb on it, there is a very good chance that it will run fine. Since a carburetor works on a pressure differential, an engine with minimum of 12 to 14 inches of manifold vacuum will usually accept a factory carburetor tune with no problem. If the camshaft brings the vacuum signal down to 12 inches of mercury or below, some internal tricks from a carburetor expert will need to be performed.

There are three areas of tuning that can be made. They are the jet size, power enrichment (metering rods in Rochester and Carter, power valve in Holley), and air bleeds. Most factory carburetors do not have easily changeable air bleeds, while an aftermarket performance design would.

The jets control how much fuel the carburetor will deliver, while the air bleeds shape the curve. Thus, if the carb was lean or rich at a given engine speed or load, an air bleed change would be required to reshape the fuel curve. If the carburetor is too rich or lean in all areas, a jet change is usually in order.

If you decide to do internal tuning on your carb, the best approach is to buy a book (there are excellent texts on every OE and aftermarket carburetor), and study it before you get into the job.

The biggest mistake an enthusiast can make is accepting a driveability problem from a carburetor-equipped engine and believing that it is normal. A properly tuned carb can run just as well as a new EFI engine–you only need to give it what it wants.


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