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Your car's carburetor (if it has one, most recent cars are FUEL INJECTED) supplies the proper mix of fuel and air to your engine. A malfunctioning carburetor can make your engine run rich (too much gas, too little air) or lean (too little gas, too much air)

Symptoms of a defective carburetor

A defective carburetor can cause smelly exhaust, smoking, hard starting, rough running, stalling, hesitation on acceleration, and poor gas mileage.

How a Carburetor works

A carburetor works by the VENTURI PRINCIPLE. If you pass air across a hole, a vacuum will be created in that hole. A carburetor has many carefully positioned holes which the air sucked in by the engine flows past. These holes are connected to the CARBURETOR BOWL, which is kept full of fuel by a FLOAT AND NEEDLE VALVE. This valve and float shut off the pressurized fuel sent from the tank by the fuel pump. It works about the same way as a toilet tank float valve. How much air flows by the carburetor venturi's is determined by the throttle plate, which restricts the airflow through the carburetor. At wide open throttle, all possible air is admitted throught he carburetor, and maximum power is gotten from the engine.

When an engine is cold it needs a richer (more fuel to air mix than usual) mixture to run correctly. A carburetor has a device called a CHOKE to accomplish this. The choke is like the throttle, except it is on the top rather than the bottom of the carb, and it closes when cold and opens when warm. When the choke is closed, it restricts the air entering the carburetor at the top, while the throttle still allows the same amount of air to go through the bottom. This means the same amount of fuel is sucked, but less air: so the mix is richer. Most carburetors have a vacuum "motor" (that is, a diaphragm moved by vacuum) that is connected to a rod that pulls the choke open a bit after the engine starts. This is called a "CHOKE PULLOFF". This enables the choke to be fully closed and provide a super rich mixture to start the engine. Once the engine starts, the choke pulloff opens the choke a bit to let the engine "get some air". If your car runs great when warmed up, but smokes black and hardly runs when you first start it, you may have a bad choke pulloff.

Common Carb Problems

Choke Pulloff

Most carburetors have a vacuum "motor" (that is, a diaphragm moved by vacuum) that is connected to a rod that pulls the choke open a bit after the engine starts. This is called a "CHOKE PULLOFF". This enables the choke to be fully closed and provide a super rich mixture to start the engine. Once the engine starts, the choke pulloff opens the choke a bit to let the engine "get some air". If your car runs great when warmed up, but smokes black and hardly runs when you first start it, you may have a bad choke pulloff.

Carburetor Float

The carburetor float is much like the float in a toilet tank. It allows fuel into the carburetor until the carburetor bowl is full, then shuts off the fuel flow. The level of fuel in the carburetor bowl is critical: too low and the carb will run lean, too high and it will run rich. On most carbs you have to take them apart to adjust the float level. An exception is some Holley carburetors. They have a screw on top of the bowl that allows you to adjust the float level on the car. A small plug is located on the side of the fuel bowl. To adjust the float level, remove this plug and start the engine. Loosen the lock nut on the top of the bowl, and adjust the screw to change the float level. The level should be even with the hole in the side of the float bowl.

How Floats Mess Up

Floats mess up in two ways: Some floats are made of foam plastic. The fuel proof outer layer of these floats can fail, and the float can saturate with gas. It then becomes a "sinker" rather than a float, and makes the fuel level in thew bowl too high. Sometimes a bad float can make gas pour out of the top of the carburetor!

A float can also stick shut, especially on an engine that has not been started for awhile. If you have gas going to the carb but there doesn't appear to be gas available in the carb, you may have a float stuck shut. In this case you can sometimes tap LIGHTLY (Carbs are made of soft metal, so don't beat the thing up!) on the top of the float bowl and the float (actually the needle valve operated by the float) will break loose and the carburetor will operate OK. Tapping on the top of the carb can also fix a float stuck open SOMETIMES, although not as often as one stuck closed.

Accelerator Pump

When you push the gas pedal down suddenly (floorboard it) like when you are passing a car, there is a device called an accelerator pump which gives a "squirt" of gas directly into the engine. This prevents hesitation or stumbling of the engine on sudden acceleration. In addition, the accelerator pump helps start the engine when cold: that's why you pump the pedal a couple of times before you start the engine. Each time you pump the pedal a squirt of fuel goes into the engine, so that's why you can flood the motor (put so much gas into it that it won't start) by pumping the pedal too much while trying to start the engine.

When accelerator pumps go bad a vehicle will hesitate on hard acceleration. It also will be hard to start, especially when cold. It's easy to check an accelerator pump: Remove the air filter and look down inside the carburetor. Have an assistant pump the gas pedal, or open and close the throttle yourself under the hood. You should see a squirt of gas going down the inside of the carburetor, or see a "cloud" of fuel vapor building up inside the carburetor throat. If the choke plate is closed you may need to hold it open with your finger or a screwdriver to see down inside the carb.

Carburetor Choke

The choke plate is at the top of the carburetor. When the engine nis warmed up fully the choke plate should be open all the way. When the engine is cold the choke plate should be closed all the way. Mo'st chokes are operated by a bi-metallic coil, which turns as it warms up. Most carburetors use either engine heat or an electric coil to heat this bi-metallic coil and open the choke. On carbs using engine heat, usually a tube runs from the exhaust manifold to the choke coil housing. Air is sucked through this tube by the carburetor into the choke coil housing, heating it up and opening the choke.

On electric chokes an electric heating coil in inside the choke coil housing and it heats the coil up, opening the choke.

A third type of choke uses engine coolant via 2 small hoses to heat the choke. Sometimes these chokes have a paraffin pellet which heats up and expands, opening the choke.

Choke Problems

Chokes can often be adjusted. On a cold engine, loosen the screws that hold the cover of the choke coil housing. Hold the throttle open a bit, and rotate the housing until the choke is closed, but just barely closed. Warm the engine up and make sure the choke opens all the way.

If the choke doesn't open all the way, it might not be getting heated enough. Check the metal tube to the exhaust manifold and make sure it's intact and not clogged. If it's an electric choke, check the wire hooked to it with a test light or volt meter to make sure it has power. The heating coil on an electric choke can burn out. Disconnect the wire to the choke and test with an ohmmeter between the terminal on the choke housing and ground. There should be some resistance, but not INFINITE resistance.

On a water heated choke, feel the side of the choke housing when the engine is warm: the housing should be warm also. If not, disconnect one of the small hoses going to the choke housing: coolaqnt should flow out of it when the engine is running. If there is no flow, the coolant supply to the small hoses may be clogged or blocked. Look for kinked or collapsed hoses. If the smalll hoses are OK, you may be able to clear out the clog by blowing compressed air through the small hoses leading ton the choke housing.

Fast Idle Cam and curb idle

A stepped cam is beside the throttle and connected to the choke mechanism. When the choke is fully closed the cam holds the throttle open a lot to keep the cold engine from stalling. As the choke opens the cam rotates, holding the throttle open less and less. Once the choke is fully open the fast idle cam doesn't hold the throttle open at all.

The "curb idle screw" adjusts the idle speed when the engine is fully warm. It holds the throttle open a bit to set the idle at the desired RPM.

If the choke never opens up all the way the carb may idle too fast, because the fast idle cam never rotates all the way to its lowest level.

Click here for some images of different carburetors

(you'd think carburetor would be spelled carburator, wouldn't you? In fact, carburator spelling gets some hits, because a lot of folks think carburator is right. Carburator is WRONG though, but if you want to spell it carburator I don't mind: or carburettor, for that matter!)
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Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.

George G. Scott, Jr.


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Auto, Car, and Truck Article List

A
ABS: Anti-Lock Brake Systems
ADVANCE: Car ignition timing
ALTERNATORS and Car Battery
AUTOMATIC TRANSMISSIONS
B
BAD CAR DESIGNS
BATTERIES: Auto, Car or Truck
BELTS AND HOSES
BEARINGS
BODY AND BUMPER REPAIRS
BRAKE REPAIRS: Car or Truck
C
Car Washing and Care
CARBURETORS:Car & Truck
CHECK ENGINE LIGHT
CLEANING: Engine Cleaning
CLUTCH REPAIRS: Car & Truck
COMPRESSION: Car Engine
COMPUTER CAR CONTROLS
CV JOINT OR CV AXLES
D
DISTRIBUTORS (IGNITION)
E
ELECTRIC WIRING REPAIR
ENGINES: Car & Truck
ENGINE CLEANING
EXPANSION PLUGS
F
FREEZE PLUGS
FUEL AIR MIXTURE
FUEL INJECTION: Car & Truck
FUEL PUMPS: Car & Truck
G
GAGES AND "IDIOT LIGHTS"
GASKETS AND SEALS
GLASS: WINDOWS AND WINDSHIELDS
H
HEADS & HEAD GASKET
HOSES AND BELTS
I
"IDIOT LIGHTS" AND GAGES
IGNITION TIMING: Car & Truck
J
AUTO JACKS: lifting cars safely
K
L
LEAN "Car runs lean"
LIGHTS: WARNING OR "IDIOT LIGHTS"
Limp Home Mode
M
MIL Light
MANUAL TRANSMISSIONS
N
NO START: Car Won't Start
O
OIL CHANGES
OIL: What's right for your car?
OIL LIGHT ON OR GAGE LOW
P
PCV Valve
Q
R
RADIATORS: Car and Truck
RICH: Car runs rich
S
SEALS AND GASKETS
SERVICE ENGINE SOON LIGHT
SPARK PLUGS
STARTERS: Auto, Truck
T
THERMOSTATS
TIMING: IGNITION TIMING
TIMING BELT & TIMING CHAIN
TIRE REPAIR
TRANSMISSIONS: AUTOMATIC
TRANSMISSIONS: MANUAL
U
V
VACUUM ADVANCE
WARNING LIGHTS OR "IDIOT LIGHTS"
Car Washing and Care
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WATER PUMP REPAIR
WINDOWS AND WINDSHIELDS
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