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Pinpointing carburetion problems

Story and photos by John Bellah

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As more vehicles today are computer-controlled and fuel-injected, the word “carburetor” is going by the wayside in the same direction of the Beta Max, dial telephone, typewriter and 8-track stereo. Bring an older car that isn’t running right into a shop manned by our younger generation and the technician may scratch his or her head and say; “That’s old technology… we’re not equipped to work on that.”

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 The two-barrel Ford carburetor was used for various applications from 221 cubic inches to more than 400 cubic inches in displacement.

The two-barrel Ford carburetor was used for various applications from 221 cubic inches to more than 400 cubic inches in displacement.

Carburetion 101

A gasoline engine runs on a mixture of gasoline and air – about one part of gasoline to about 12 parts of air. Notice I said “about,” as there are several variables. Today’s computer-controlled engines do this automatically. The computer takes inputs from oxygen sensors and other engine data, then signals the fuel injection and ignition to inject the proper amount of fuel and ignite it at the precise time. Today’s cars run well, produce few emissions and are more fuel-efficient than what our parents and grandparents drove.

Older vehicles use carburetors, which also do a pretty fair job of providing a fuel and air mixture – when they work correctly. If not, the vehicle may not run properly, or may not run at all. A carburetor has several different circuits and, depending on the make and design of the carburetor, different methods may be utilized in accomplishing these needed tasks. If one is intending to work on a specific carburetor, referring to the overhaul sheet (which is included with a repair or overhaul kit) or reading a shop manual would be very helpful.

If the engine is not running right, the carburetor is the last thing that should be checked before blindly tearing into it. Ensure that compression is up to par in all of the cylinders, the valves are adjusted to their set clearances and the ignition system is operating properly. Often a faulty spark plug, wire, sticking ignition points or sticking, improperly-adjusted or leaking valves can mistakenly be diagnosed as carburetor trouble.

Carburetion problems can be boiled down to no fuel at all, not enough fuel (lean), too much fuel (rich) or fuel delivered to only half of the cylinders.

No Fuel at All While the engine is off, a quick check requires removing the air cleaner for a look down the primary barrel(s) and to operate the throttle. If there is fuel in the float bowl, and assuming the accelerator pump is operating properly, there should be a spray of gasoline squirted into the venturi every time the throttle is opened.Working backwards, if there is no fuel in the float bowl and the vehicle has been sitting for a long period of time, the gasoline may have evaporated and left varnish, which has clogged the inlet needle valve. Pull the fuel line and crank the engine. If no fuel comes out, then possibly the fuel pump isn’t working or the fuel filters are clogged. Before looking at the fuel pump, make sure there is gasoline in the tank.Another reason for poor fuel delivery on older vehicles is a pinhole leak in the fuel line. A good place to start would be to check the neoprene hose connector between the tank outlet and the fuel line. Often, over time, that 2-inch connector can be the culprit. Additionally, the steel fuel line may have developed rust and corrosion both internally or externally or may have chafed against a frame member. When in doubt, replace the fuel line.The fuel tank itself can be a source of problems. Sitting for long periods can develop rust and/or varnish. Foreign objects from acts of vandalism, such as sugar, or pieces of Styrofoam, cork or wood can also cause their share of problems. The body designs of some cars can collect large amounts of water if the vehicle has sat outside for long periods of time during the rainy season.While on the subject of fuel tank contaminates, the myth that sugar will ruin an engine by freezing it solid is just that — an old wives’ tale. Both the government and the insurance industry have extensively tested sugar contamination of fuel tanks and all it does is clog the fuel system. Styrofoam also will clog fuel systems. Many older cars have a drain plug at the bottom of the fuel tank. Periodically drain a quart or so from the tank. This will remove water, which settles at the bottom, along with dirt and sediment. If large amounts of rust comes out, consider removing the tank for cleaning or replacement. If the tank is serviceable, most radiator shops can clean out a contaminated fuel tank. Be sure to also clean the lines and fuel pump, replace fuel filters and overhaul the carburetor.

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 Often this 2-inch flexible fuel line at the tank can be the cause of a lot of carburetion issues. As it deteriorates and cracks, it will leak. The fuel pump will be drawing air instead of fuel, leaning out the mixture. Eventually you will have a “no start” situation.

Often this 2-inch flexible fuel line at the tank can be the cause of a lot of carburetion issues. As it deteriorates and cracks, it will leak. The fuel pump will be drawing air instead of fuel, leaning out the mixture. Eventually you will have a “no start” situation.

While concentrating with the fuel tank, often there is a mesh filter covering the tank pick-up, which on some vehicles is part of the fuel tank sending unit. Over a period of years, this filter could become clogged or may be completely deteriorated. While the sending unit is removed, check and see if the float is fuel-logged. If it is, that explains why the fuel gauge reads “E.”

Too Lean, Too Rich

As previously stated, a gasoline engine requires about a 12:1 air-fuel ratio. That is a very loose generality as different engine needs require the mixture be tailored. There are numerous variables. Cold-starting and warm-up require a richer mixture, which is usually handled by a choke butterfly that limits the amount of air into the carburetor until the engine is at operating temperature. Acceleration, heavy loads and, in some cases, idling also require a richer mixture. Cruise requires a leaner mixture for fuel economy. The trend towards exhaust emission controls in the 1960s also calls for leaner mixtures. Altitude also makes a difference (e.g., the higher you climb, the leaner you need to jet).

An engine that is running lean may be hesitating. A too-lean condition may also cause overheating, and prolonged operation will cause engine failure – at minimum, burned valves with the added possibility of burning holes in pistons.

An excessively rich mixture can eventually cause serious engine damage, aside from excessive fuel consumption, as the rich mixture will soon wash the lubricating oil from the cylinders and prematurely wear cylinder bores and pistons and rings. Symptoms of a rich mixture can be misfiring and stalling at idle. Copious amounts of black smoke from the exhaust pipe are a tip-off to this condition. The best way to check your fuel mixture is to use an exhaust analyzer, which most tune-up and emission-control shops have. If an exhaust analyzer isn’t readily available, “reading” spark plugs after a run of several miles can give a ballpark figure. Proper mixtures will leave the spark-plug insulators a medium tan/brown. Too rich is black and too lean, the insulators will show white.

Vapor lock can be a problem, especially with today’s “reformulated” gasoline. Another potential problem can occur while driving in hot weather with gasoline that has been blended for winter operation. Today’s “reformulated” gasoline, also termed E-10, is 90 percent gasoline with about 10 percent alcohol for lower emissions. Some older engines don’t like E-10 and may be prone to vapor lock as the alcohol evaporates faster than gasoline. Additionally, gasoline is blended to suit the area where it is sold, keeping in mind the variables – ambient temperature and altitude. Gasoline sold in high-altitude regions is blended with lower octane ratings as combustion pressures aren’t as high in higher elevations.

Sometimes a change in gasoline brand may cure a vapor lock problem. Other times, more drastic measures may be needed, such as insulating the carburetor using extra gaskets or insulating the fuel line. If you have a “correct” collector car with vapor lock issues, an old “shade-tree” solution is to place several wood clothes pins on the fuel line. The clothes pins act as a heat-sink to dissipate the heat. Another temporary fix would be to wrap the fuel line in aluminum foil or similar non-flammable insulating material. Upon arrival at the car show or cruise night, these shade-tree fixes can be temporarily removed to maintain the “correctness.”

A stuck manifold heat-control valve, or commonly called the heat-riser, can cause warm-up and cold drivability problems. If this valve is stuck in the cold position, it will constantly direct hot exhaust gasses under the carburetor. This can cause hard starting when hot, as the fuel will have boiled out of the float bowl.

A blocked exhaust system can mimic carburetion problems due to the lack of power, but the loud hiss from underneath the car from exhaust gasses trying to escape is the giveaway. Testing with a vacuum gauge will confirm a blocked exhaust system. Hook up the vacuum gauge and rev the engine to about 3,000 rpm. If the needle of the vacuum gauge slowly sinks towards zero, there is a blockage of the exhaust system.

Some engines, over a period of time, can have the heat passage under the carburetor become completely blocked. This may be fine on a racing engine where drivers completely block off the heat passages for maximum horsepower. On a street engine, this will cause excessive fuel consumption and drivability problems in cold weather. If the heat passage is clogged, the intake manifold needs to be removed and the passages cleaned out. Small-block Chrysler engines were especially prone to this condition.

Many vehicles use vacuum to operate different vehicular components such as power brake boosters, windshield wipers, headlamp doors, radio antennas, trunk releases, heater and air conditioning valves and passage doors. A vacuum leak will cause a rough idle and sometimes it can be tricky to spot through the seemingly endless maze of hoses of cars of the ’60s and ’70s. Don’t overlook the fact that the manifold itself may be the culprit, as it could be warped, cracked or has developed pinhole leaks through casting flaws. A pair of pliers and a can of spray carburetor cleaner can be useful in tracking down stray vacuum leaks.

A rough idle while sitting at a stop light with the brakes applied may indicate the power brake booster is faulty. While the engine is idling, use the pliers to pinch off each vacuum hose one by one. If the idle changes, you have at least narrowed down the problem. Spray the carburetor cleaner around the suspected area and if the idle changes, you are near the problem. Keep in mind, over time and mileage, throttle shaft bushings can wear, allowing unwanted air into the carburetor.

Float System 

Fuel enters the carburetor under pressure from the fuel pump and enters the float bowl. Either a hollow or, in some cases, a plastic float will maintain the proper fuel level inside the carburetor. This is similar to a float system inside your toilet tank. When the float rises, it signals to the needle and seat to either allow fuel inside or to shut it off. It is important for the float to be working right and properly adjusted. Some floats are made of brass and they are hollow. Occasionally, they may develop a pinhole leak and be fuel-logged – thus work improperly. Flooding out or stalling during a sharp turn may indicate there is a float problem.

Many vehicles manufactured in the 1970s and later may have a plastic float which, over time, may also become fuel-logged. A fuel-saturated float, once removed from the carburetor and placed on a work bench can become deceptive as the fuel will quickly evaporate and it will weigh as much as a new float. If one suspects the float is saturated, compare the float weights immediately when it is removed from the carburetor so the fuel doesn’t evaporate. Cheap insurance would be to automatically replace plastic floats if they have been in service for more than two years. Leaking and/or sticking needle and seat valves can also cause problems, such as flooding.

Occasionally, if a vehicle has been parked on a steep hill, pointed downward, it can develop a hydrostatic lock. This is when the fuel (usually with a full fuel tank) develops so much pressure it will bypass the needle and seat and flood the engine. The result is a completely locked-up engine. Do not attempt to start an engine with this condition. Gasoline does not compress and the result can be severe engine damage. Repair the carburetor, remove the spark plugs and ensure the cylinders are free of any liquid gasoline. An engine that has gone through this ordeal should also have the oil changed as gasoline could enter the crankcase and dilute and contaminate the engine oil.

Low-High Speed Systems

From the float chamber, fuel is metered through what is termed a main jet, which regulates the amount of fuel to the other circuits. A single-barrel carburetor has one main jet. Two-barrel carburetors will have two main jets and a four-barrel carburetor will have four main jets. The main jets are precision-drilled, and in the case of a four-barrel carburetor, the secondary jets are often much larger than the primary jets. Pay careful attention and do not interchange the primary with secondary main jets. You will quickly foul spark plugs and you will have poor fuel mileage.

On some carburetors, additional mixture regulation can be done by metering rods, which are stepped or tapered rods that pass through the main jet(s). Metering rods are vacuum-controlled and regulate the mixture – richer or leaner as to engine need. Metering rods must be straight to work properly. Some people are tempted to drill out main jets for more speed and power. This is not a good idea as they are precision-drilled (and numbered) at the factory. Drilling out a jet may often impede the flow of fuel as drilling alters the shape of the jet, thus the fuel flow.

Other carburetors rely on power-circuits, economizer and/or power valves to tailor the fuel mixture to engine demands. On Holley carburetors, the power valves can easily be blown out due to an engine backfire.

A quick way to test an accelerator pump is to remove the air cleaner assembly and look down the primary throat(s) of the carburetor and open the throttle linkage. If the accelerator pump is working properly, you will see the pump shoot out a shot of fuel. Only do this while the engine is off, however. If it is a four-barrel carburetor, only the primary barrels, which are closest to the front of the engine, have an accelerator pump. 

Accelerator Pump Circuit

Basic laws of physics dictate that air moves faster than fluid. Opening the throttle will open the throttle plates and will cause a lot of air to enter the engine. To overcome the lag of fuel delivery, there is an accelerator pump that squirts fuel into the venturi, or bore of the carburetor. If the accelerator pump does not operate properly, the engine may not start or will stumble badly on acceleration.

On some carburetors, the accelerator pump is made of a leather plunger with an expanding spring to keep the leather in contact with the bore. Over time, the leather plunger may become worn or will dry up. A temporary shade-tree fix for a faulty accelerator is to remove the accelerator pump plunger and, with a small screwdriver, carefully expand the spring. Lubricate the leather plunger with Vaseline before reinstalling it.

Another thing to check when the accelerator pump isn’t operating correctly is to see if the check balls are properly in place, as the last person who opened up the carburetor may have not replaced them properly. This is where the shop manual or carburetor overhaul sheet will come in handy. If the check balls are in place, ensure they are not corroded and in their bores, or gummed up with varnish.

Idle Circuit

Most carburetors have a separate idle circuit and, compared to today’s fuel-injected engines which are pre-set, the idle speed and mixture can actually be adjusted. Often, dirt in the idle circuit can cause a no-idle situation, where the engine will run well at road speed. Sometimes, removing the idle mixture screw, blowing compressed air into the hole, replacing the mixture screw and readjusting the mixture may cure this problem.

Often, a poor idle condition will not be the carburetor’s fault, but a clogged Positive Crankcase Ventilation (PCV) system. With crankcase blow-by gasses unable to escape, idle will deteriorate and, under severe conditions, pressure will force oil past the seals.


Depending on the vehicle, the choke can be operated manually or automatically. A cold engine requires a richer mixture and the choke butterfly reduces the amount of air until the engine reaches operating temperature. With the various choke systems, a whole chapter could be written just on carburetor chokes alone. When dealing with an older vehicle, remember, literally dozens of mechanics and so-called mechanics may have worked on the vehicle. Especially with exhaust emission controls coming about in the mid-1960s, the result was many emission-controlled vehicles ran poorly until reaching operating temperatures. Many gas station and shade-tree mechanics, in an attempt to keep the customer happy, would “modify” the choke system by bypassing or modifying some of the linkages. This is where the shop manual or carburetor overhaul instruction sheet will come in handy. Start with recommended settings and go from there.

Some Final Words

Does your vehicle have the right carburetor? There are many variations, such as fleet, taxi, police, high-performance and economy applications; the type of PCV system (California or 49-state); what emission control package (California, California high altitude, Federal or Federal high altitude); and accessories (air conditioning, power steering and power brakes). Some vehicles may even utilize different carburetor manufacturers for the same engine. Carburetors are usually identified by a series or model number that is either stamped somewhere on the carburetor body, or on a tag.

Sometimes, however, the tags are discarded during the overhaul process or the numbers are deliberately ground off in the rebuilding process. In these cases, it may be impossible to properly identify the carburetor. And many carburetors do appear to be identical on the outside, meaning that by outward appearance, it could fit a range of engines from a little over 200- to over 400-cubic-inch engines.

If you are having a carburetor rebuilt by a rebuilder, use a quality rebuilder who will replace throttle shaft bushings, along with other critical parts, such as check valves, and flow-test the carburetor. Yes, you will pay more than the cheapie outfits who use minimum-wage employees who just mix and match parts, throw a cheap gasket kit and shove it out the door. “Well, it looks to be the right one for your car.”

Junk yard and swap meet carburetors can be another “pig in a poke.” What is the history of the carburetor? Does it have lean jetting as it was used in high altitude areas, or has it been hot-rodded by an amateur who left out half of the pertinent seals and hogged out the jets under the theory: “If some is good, more must be better!”

Occasionally, one might find a V-type engine that is only running on half of the cylinders. If one bank of cylinders is dead, and it has a dual exhaust system, there is a strong possibility that one side of the exhaust system is blocked. Early-1960s Lincolns were known to have this problem, as the exhaust crossover was behind the mufflers. If half the cylinders, in no particular order, are dead, it is possible there is a problem with one side of the carburetor.

 The Rochester Carburetor was popular on Chevrolet 6-cylinder engines.

The Rochester Carburetor was popular on Chevrolet 6-cylinder engines.

Individual Carburetor Tips

With the high prices of gasoline these days, some may feel that changing over from a four-barrel to a two-barrel carburetor will be the ticket for better fuel mileage. In most situations, that is not the case. Carburetors are a compromise – especially a two-barrel carburetor, which must be economical, yet flow enough fuel and air mixture to sustain the engine at high speed.

This is where the four-barrel carburetor comes in. In most four-barrel carburetors, the front two barrels are the primary barrels and their venturis are small enough to provide good fuel atomization for low-speed driving. The rear two barrels are secondary barrels that will only come in at 3/4 and wider throttle for maximum performance. Unless one drives with their foot to the floor all the time, the four-barrel will get better mileage and have the performance when needed, such as passing another vehicle.

Two- and four-barrel Holley carburetors are unique, as the float bowls are bolted on to the main body of the carburetor. There are all kinds of parts available for Holleys and they can be tricked out. The float levels can be checked and adjusted while on the car. Excessive heat, such as a stuck heat riser, can warp the metering blocks, which will cause all kinds of carburetor gremlins as fuel is going to the wrong circuits. Cheap insurance is to insure that the float bowls are snug, but be careful to not over-tighten as this will strip the threads in the pot-metal housing.

While many people swear at the Carter Thermo-Quad carburetor, like any other carburetor, they have to be set up properly and they will work right. Part of the problem is many did not understand the Thermo-Quad and many were junked prematurely.

With a Thermo-Quad, the float levels must be set exactly to specifications. While early models of this carburetor had brass floats, later versions came with plastic floats, which can become fuel-saturated after a couple of years.

Premature accelerator pump wear can be corrected by lightly smoothing out the bore surfaces with fine emery paper. Another issue is there are two small “O” rings in the float bowl that are vitally necessary to be in place for the carburetor to work properly. If they are not in place, the engine will flood and never run properly.

Thermo-Quad carburetors were constructed with a plastic float chamber, which, in theory, kept the fuel cooler and denser, for more power. Over time, these float bowls may warp. Improper assembly may crack the bowls. Once the carburetor is assembled, visually check if everything is properly aligned and seated. Then, with the carburetor in a level position, drop in each assembly screw one by one. If the screws easily travel down to meet the threads at the base, the carburetor components are properly aligned. If they hang up, something is wrong. Re-check the alignment of the components.

Ensure the correct carburetor base gasket is used and is properly in place. Some carburetors have a power circuit triggered when intake manifold vacuum falls below 10 inches. The vacuum inlet is often in the carburetor base and if the gasket covers the inlet, the engine will be permanently running rich. Six-cylinder Chevrolets were prone to this condition.

Finally, do not overtighten the air filter. This can warp the top of the carburetor, which can affect the choke and carburetor performance.

The author, John Bellah, spent his early adulthood specializing in carburetion, tune-up and electrical issues on both domestic and foreign vehicles in the early to mid 1970s.


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