other than a bad carburetor what can cause gas to leak out the exhaust

Story and photos by John Bellah

As more vehicles today are reckoner-controlled and fuel-injected, the word "carburetor" is going by the wayside in the same direction of the Beta Max, punch 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 caput and say; "That'due south old technology… we're not equipped to work on that."

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

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 in that location are several variables. Today'southward computer-controlled engines do this automatically. The calculator takes inputs from oxygen sensors and other engine data, and then signals the fuel injection and ignition to inject the proper amount of fuel and ignite it at the precise time. Today'due south cars run well, produce few emissions and are more fuel-efficient than what our parents and grandparents drove.

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

If the engine is non running right, the carburetor is the last thing that should be checked before blindly tearing into it. Ensure that compression is upwards to par in all of the cylinders, the valves are adjusted to their prepare clearances and the ignition system is operating properly. Oftentimes a faulty spark plug, wire, sticking ignition points or sticking, improperly-adjusted or leaking valves tin can mistakenly be diagnosed equally carburetor trouble.

Carburetion problems tin 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 await downwards the primary barrel(due south) and to operate the throttle. If there is fuel in the float basin, 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 in that location is no fuel in the float bowl and the vehicle has been sitting for a long menses of fourth dimension, 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, and then maybe the fuel pump isn't working or the fuel filters are clogged. Before looking at the fuel pump, make certain 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 identify to kickoff would be to check the neoprene hose connector between the tank outlet and the fuel line. Often, over fourth dimension, that ii-inch connector tin can be the culprit. Additionally, the steel fuel line may have adult rust and corrosion both internally or externally or may have chafed against a frame fellow member. When in dubiety, supplant the fuel line.The fuel tank itself can be a source of problems. Sitting for long periods tin develop rust and/or varnish. Foreign objects from acts of vandalism, such equally carbohydrate, or pieces of Styrofoam, cork or wood tin as well cause their share of issues. The body designs of some cars can collect big amounts of h2o if the vehicle has sat outside for long periods of fourth dimension 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 authorities and the insurance industry accept 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 lesser of the fuel tank. Periodically drain a quart or so from the tank. This will remove h2o, which settles at the lesser, 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 besides make clean the lines and fuel pump, supervene upon fuel filters and overhaul the carburetor.

Often this 2-inch flexible fuel line at the tank tin be the cause of a lot of carburetion problems. Equally it deteriorates and cracks, information technology will leak. The fuel pump will exist drawing air instead of fuel, leaning out the mixture. Eventually y'all 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 role of the fuel tank sending unit. Over a menstruation of years, this filter could become clogged or may be completely deteriorated. While the sending unit is removed, cheque and see if the float is fuel-logged. If it is, that explains why the fuel gauge reads "Due east."

Too Lean, Likewise Rich

As previously stated, a gasoline engine requires about a 12:1 air-fuel ratio. That is a very loose generality as dissimilar engine needs require the mixture be tailored. In that location are numerous variables. Cold-starting and warm-upwards 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. Dispatch, heavy loads and, in some cases, idling also require a richer mixture. Prowl requires a leaner mixture for fuel economy. The tendency towards exhaust emission controls in the 1960s also calls for leaner mixtures. Altitude too makes a difference (eastward.g., the higher you climb, the leaner yous demand to jet).

An engine that is running lean may be hesitating. A also-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 tin can eventually crusade serious engine damage, bated from excessive fuel consumption, as the rich mixture will shortly launder the lubricating oil from the cylinders and prematurely habiliment cylinder bores and pistons and rings. Symptoms of a rich mixture can exist misfiring and stalling at idle. Copious amounts of black smoke from the exhaust piping are a tip-off to this condition. The best way to check your fuel mixture is to utilise an exhaust analyzer, which most melody-upward and emission-control shops have. If an exhaust analyzer isn't readily available, "reading" spark plugs later on a run of several miles can give a ballpark figure. Proper mixtures will go out the spark-plug insulators a medium tan/chocolate-brown. Too rich is black and too lean, the insulators volition show white.

Vapor lock can exist a problem, especially with today'due south "reformulated" gasoline. Another potential problem tin can occur while driving in hot weather with gasoline that has been blended for winter functioning. Today'due south "reformulated" gasoline, besides termed E-10, is xc percent gasoline with most 10 percent booze for lower emissions. Some older engines don't like East-10 and may be decumbent to vapor lock as the alcohol evaporates faster than gasoline. Additionally, gasoline is blended to adjust the area where information technology is sold, keeping in listen the variables – ambient temperature and distance. Gasoline sold in high-distance regions is composite with lower octane ratings equally combustion pressures aren't as high in higher elevations.

Sometimes a change in gasoline make may cure a vapor lock problem. Other times, more desperate measures may be needed, such equally insulating the carburetor using extra gaskets or insulating the fuel line. If you have a "correct" collector car with vapor lock bug, an old "shade-tree" solution is to place several wood clothes pins on the fuel line. The wearing apparel pins act as a heat-sink to dissipate the oestrus. Another temporary fix would be to wrap the fuel line in aluminum foil or similar non-flammable insulating material. Upon arrival at the car testify or prowl nighttime, 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 crusade warm-upwardly and cold drivability problems. If this valve is stuck in the cold position, information technology volition constantly straight hot exhaust gasses under the carburetor. This tin can crusade hard starting when hot, as the fuel volition accept boiled out of the float bowl.

A blocked exhaust system tin mimic carburetion problems due to the lack of power, merely the loud hiss from underneath the automobile from exhaust gasses trying to escape is the giveaway. Testing with a vacuum gauge will confirm a blocked exhaust organisation. Hook up the vacuum gauge and rev the engine to about 3,000 rpm. If the needle of the vacuum guess slowly sinks towards zippo, in that location is a blockage of the exhaust system.

Some engines, over a period of fourth dimension, can have the heat passage nether 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 volition crusade excessive fuel consumption and drivability problems in common cold weather. If the oestrus passage is clogged, the intake manifold needs to be removed and the passages cleaned out. Small-block Chrysler engines were especially prone to this status.

Many vehicles employ 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 tin be tricky to spot through the seemingly countless maze of hoses of cars of the '60s and '70s. Don't overlook the fact that the manifold itself may exist the culprit, as information technology could be warped, cracked or has adult pinhole leaks through casting flaws. A pair of pliers and a can of spray carburetor cleaner can exist useful in tracking down devious 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 past 1. If the idle changes, yous have at least narrowed down the trouble. Spray the carburetor cleaner around the suspected surface area and if the idle changes, you lot are near the trouble. Keep in heed, over time and mileage, throttle shaft bushings can article of clothing, assuasive unwanted air into the carburetor.

Float System

Fuel enters the carburetor under pressure level 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, information technology signals to the needle and seat to either let fuel within or to close it off. It is important for the float to exist working right and properly adjusted. Some floats are made of contumely and they are hollow. Occasionally, they may develop a pinhole leak and be fuel-logged – thus piece of work improperly. Flooding out or stalling during a sharp plow may bespeak in that location is a float problem.

Many vehicles manufactured in the 1970s and later on may have a plastic bladder which, over time, may also become fuel-logged. A fuel-saturated bladder, in one case 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 accept been in service for more than 2 years. Leaking and/or sticking needle and seat valves can also crusade problems, such as flooding.

Occasionally, if a vehicle has been parked on a steep loma, pointed downward, information technology can develop a hydrostatic lock. This is when the fuel (ordinarily with a full fuel tank) develops and so much pressure it will bypass the needle and seat and overflowing the engine. The result is a completely locked-up engine. Do non attempt to start an engine with this condition. Gasoline does not compress and the result can be astringent 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 likewise have the oil changed equally 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 master jet, which regulates the amount of fuel to the other circuits. A single-barrel carburetor has one main jet. 2-barrel carburetors volition have ii main jets and a four-butt carburetor will take 4 main jets. The main jets are precision-drilled, and in the case of a four-barrel carburetor, the secondary jets are frequently much larger than the primary jets. Pay conscientious attention and do not interchange the primary with secondary main jets. Y'all will rapidly foul spark plugs and you will have poor fuel mileage.

On some carburetors, additional mixture regulation can be done past metering rods, which are stepped or tapered rods that pass through the main jet(southward). 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 than speed and ability. This is non a good idea as they are precision-drilled (and numbered) at the mill. Drilling out a jet may often impede the period 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 hands exist blown out due to an engine backlash.

A quick manner to exam an accelerator pump is to remove the air cleaner assembly and look downwardly the primary pharynx(s) of the carburetor and open the throttle linkage. If the accelerator pump is working properly, you volition come across the pump shoot out a shot of fuel. Only do this while the engine is off, still. If it is a four-barrel carburetor, only the principal barrels, which are closest to the forepart of the engine, take an accelerator pump.

Accelerator Pump Excursion

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, in that location is an accelerator pump that squirts fuel into the venturi, or diameter of the carburetor. If the accelerator pump does not operate properly, the engine may not start or will stumble desperately on acceleration.

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

Some other 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 upward the carburetor may have not replaced them properly. This is where the shop transmission or carburetor overhaul sheet will come in handy. If the cheque balls are in place, ensure they are not corroded and in their bores, or gummed upward with varnish.

Idle Circuit

Most carburetors have a separate idle circuit and, compared to today's fuel-injected engines which are pre-gear up, the idle speed and mixture can really 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 spiral, bravado compressed air into the hole, replacing the mixture screw and readjusting the mixture may cure this problem.

Frequently, a poor idle condition volition not be the carburetor'due south mistake, but a clogged Positive Crankcase Ventilation (PCV) system. With crankcase blow-by gasses unable to escape, idle will deteriorate and, under severe conditions, pressure level will force oil past the seals.

Choke

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 affiliate could be written only on carburetor chokes alone. When dealing with an older vehicle, call up, literally dozens of mechanics so-chosen mechanics may have worked on the vehicle. Especially with frazzle emission controls coming virtually in the mid-1960s, the outcome was many emission-controlled vehicles ran poorly until reaching operating temperatures. Many gas station and shade-tree mechanics, in an attempt to continue the customer happy, would "modify" the choke organization by bypassing or modifying some of the linkages. This is where the shop manual or carburetor overhaul instruction sheet will come in handy. Starting time with recommended settings and go from at that place.

Some Final Words

Does your vehicle accept the correct carburetor? There are many variations, such as fleet, taxi, police force, high-performance and economy applications; the type of PCV system (California or 49-state); what emission control package (California, California loftier altitude, Federal or Federal high distance); and accessories (air-conditioning, ability steering and power brakes). Some vehicles may even utilise different carburetor manufacturers for the aforementioned 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, yet, the tags are discarded during the overhaul process or the numbers are deliberately footing off in the rebuilding process. In these cases, information technology may be impossible to properly identify the carburetor. And many carburetors do announced to exist identical on the outside, pregnant that by outward appearance, information technology 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 supercede throttle shaft bushings, along with other disquisitional parts, such equally check valves, and catamenia-test the carburetor. Yes, you volition pay more than the cheapie outfits who use minimum-wage employees who simply 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 bandy come across carburetors tin be another "pig in a poke." What is the history of the carburetor? Does information technology take lean jetting as information technology was used in high altitude areas, or has information technology been hot-rodded past an amateur who left out half of the pertinent seals and hogged out the jets under the theory: "If some is practiced, more than must be better!"

Occasionally, 1 might find a 5-type engine that is only running on one-half of the cylinders. If ane bank of cylinders is dead, and information technology has a dual exhaust arrangement, there is a potent possibility that ane side of the exhaust system is blocked. Early-1960s Lincolns were known to have this problem, as the exhaust crossover was backside the mufflers. If half the cylinders, in no item social club, are dead, it is possible in that location is a problem with i side of the carburetor.

The Rochester Carburetor was popular on Chevrolet half-dozen-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-butt 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 plenty fuel and air mixture to sustain the engine at loftier speed.

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

Ii- and four-barrel Holley carburetors are unique, as the float bowls are bolted on to the main trunk of the carburetor. At that place 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 equally a stuck rut riser, tin warp the metering blocks, which volition cause all kinds of carburetor gremlins as fuel is going to the wrong circuits. Cheap insurance is to insure that the bladder bowls are snug, but exist conscientious to not over-tighten as this volition 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 exist prepare properly and they will work correct. Part of the trouble is many did not sympathise the Thermo-Quad and many were junked prematurely.

With a Thermo-Quad, the float levels must exist set exactly to specifications. While early models of this carburetor had contumely floats, subsequently versions came with plastic floats, which tin become fuel-saturated afterwards a couple of years.

Premature accelerator pump wear can exist corrected by lightly smoothing out the bore surfaces with fine emery paper. Another issue is there are 2 small-scale "O" rings in the float bowl that are vitally necessary to be in place for the carburetor to work properly. If they are non in place, the engine will flood and never run properly.

Thermo-Quad carburetors were synthetic with a plastic float bedroom, which, in theory, kept the fuel cooler and denser, for more ability. Over time, these bladder bowls may warp. Improper associates may crevice the bowls. In one case the carburetor is assembled, visually check if everything is properly aligned and seated. So, with the carburetor in a level position, drop in each assembly screw one past i. If the screws easily travel downward to run into the threads at the base of operations, the carburetor components are properly aligned. If they hang up, something is wrong. Re-cheque the alignment of the components.

Ensure the correct carburetor base gasket is used and is properly in place. Some carburetors have a power excursion 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 tin can warp the top of the carburetor, which can affect the asphyxiate and carburetor performance.

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

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