A carburetor uses intake vacuum to supply fuel to the engine. As air is pulled down through the throat of the carburetor by intake vacuum, fuel is siphoned from the carburetor's fuel bowl and mixed with the incoming air to form a combustible mixture. At idle, the fuel enters the carburetor throat through one or small small idle ports just above the throttle plate. At higher engine speeds, fuel is pulled through the main metering jets into the venturi (the narrowest part of the carburetor throat). The air/fuel mixture then flows down through the intake manifold and into the cylinders where it is burned to produce power. Though the basic operation of a carburetor is fairly simple, it also relies on a number of add-on devices for cold starting, idle control and emissions. Changes in emission regulations in the early 1980s made carburetors obsolete because they were unable to meet the new emission requirements. By the mid-1980s, carburetors were history on new production vehicles, having been replaced by throttle body and multiport electronic fuel injection systems. Carburetor Problems When a carburetor is clean and is working properly, the engine should start easily (hot or cold), idle smoothly, and accelerate without stumbling. The engine should get normal fuel economy and emissions should be within limits for the year of the vehicle. Problems that are often blamed on a "bad" or "dirty" carburetor include hard starting, hesitation, stalling, rough idle, flooding, idling too fast and poor fuel economy. Sometimes it is the carburetor and sometimes it is something else. Carburetors can be tricky to rebuilt, and expensive to replace, so you want to be sure of your diagnosis before you touch this critical part. A choke is necessary for cold starting to richen the Air/Fuel mixture and increase idle speed while the engine is warming up. Hard Cold Starting Engine Problems Hard starting can be caused by a choke that fails to close and causes a rich fuel mixture when the engine is cold. But there's no need to rebuild or replace the carburetor if all that's needed is a simple adjustment or cleaning of the choke mechanism and linkage. Chokes are very sensitive, and easily misadjusted (which is why the government required the auto makers to make choke and idle mixture adjustments "tamper-resistant" in the 1980s). Inside the choke housing is a coiled bi-metal heat-sensing spring that contracts when it cools and expand (unwinds) when it gets hot. The spring opens and closes the choke plate on top of the carburetor. The spring is inside a black plastic choke housing on the top or side of the carburetor. The spring is heated by an electric heating element inside the cover and/or heat from the exhaust manifold that is siphoned up into the housing through a small metal tube. If the heating coil has burned out or is not receiving voltage, or the heat riser is plugged with rust, loose or missing, the choke will not warm up properly. This will cause the choke to say on all the time, or too long, making the engine run rich and idle too fast. If the bi-metal choke spring is broken, the choke will never close. A cold engine needs a very rich mixture to start, so if the choke isn't working it will suck too much air. A broken choke will also prevent the engine from idling properly (no fast idle while it is warming up) which can cause it to stall until it reaches normal operating temperature. If the shaft that opens and closes the choke is dirty, it may cause the choke to stick. The same goes for the choke linkage if it is dirty or damaged. Even if the choke is defective, a choke repair kit or a new bimetal spring should be all that's necessary to eliminate the starting problem. Replacing the entire carburetor is unnecessary and is the same as replacing the engine because the water pump is bad. Other causes of hard starting include vacuum leaks, ignition problems (worn or dirty spark plugs, bad plug wires, cap, rotor, etc.), low compression, even a weak starter or battery. Hard Hot Starting Engine Problems As for hot starting problems, the carburetor is seldom to blame. A hot start condition is usually the result of too much heat in the vicinity of the carburetor, fuel lines or fuel pump. Heat causes the fuel in the fuel lines, carburetor bowl or pump to boil. This creates a "vapor lock" condition which can make a hot engine hard to start. Replacing or rebuilding the carburetor wouldn't solve anything because the real culprit is heat. What needs to be done here is to reroute the fuel line away from sources of heat (like the exhaust manifold and pipe), and/or to insulate the fuel line by fabricating aheat shield or wrapping the fuel line with insulation. Hot start problems can also be caused by excessive resistance in a starter, poor battery cable connections, or a faulty ignition module that acts up when it overheats. Engine Hesitation or Stumble When Accelerating Hesitation is a classic symptom of a lean fuel mixture (too much air, not enough fuel) and can be caused by a dirty or misadjusted carburetor, or one with a weak accelerator pump or worn throttle shafts. Rebuilding or replacing the carburetor may be necessary. The accelerator pump squirts and extra dose of fuel into the throat of the carburetor when the throttle opens. This helps offset the extra gulp of air that is sucked in until fuel flow through the metering circuits can catch up to the change in air velocity through the venturi (the narrow part of the carburetor throat). The accelerator pump may use a rubber diaphragm or a rubber cup on a piston to pump fuel through its discharge nozzles. If the diaphragm is torn or the piston piston seal is worn, the accelerator pump may not deliver it's normal dose of fuel. Or, if the discharge nozzles are plugged with dirt or fuel varnish deposits, it can restrict fuel flow. The operation of the accelerator pump can be checked by removing the air filter, looking down into the carburetor, and pumping the throttle. You should see a jet of fuel squirt into each of the front venturis (barrels) of the carburetor. If no fuel squirts out, or the stream is very weak, or only one of the two discharge nozzles on a two-barrel or four-barrel carburetor are working, the accelerator pump circuit has a problem. Fuel usually enters the accelerator pump past a one-way steel check ball. The ball lets fuel in, but is pushed back against its seat by pressure inside the pump when the throttle opens. If this check ball is stuck open, it acts like a pressure leak and prevents the accelerator pump from squirting fuel through the discharge nozzles. If the check ball is stuck shut, it will prevent fuel from entering the pump and there will be no fuel to pump through the discharge nozzles. If the carburetor jets are coated with fuel varnish deposits, or there is dirt inside the fuel bowl, this can restrict the flow of fuel causing a lean condition. Cleaning the carburetor with carburetor cleaner can get rid of the dirt and varnish deposits to restore normal operation. Air leaks elsewhere on the engine can also lean out the fuel mixture. Air can enter the intake manifold through loose or cracked vacuum hoses, emission hose or the PCV system. Vacuum leaks in the carburetor base gasket or insulator, intake manifold gaskets, power brake booster or other vacuum accessories can admit unwanted air. Air can even get into the manifold past badly worn valve guides and seals. A defective EGR valve that fails to close at idle or when the engine is cold can be another cause of hesitation. Other causes may include a defective distributor advance mechanism, a weak ignition coil, carbon tracks on the coil tower or distributor cap, bad plug wires, worn or dirty spark plugs that misfire when the engine is under load, or even an exhaust restriction. Even bad gas can cause hesitation problems. So before the carburetor is rebuilt or replaced, these other possibilities need to be investigated an ruled out. Engine Hesitation Under Load A hesitation, stumble or misfire that occurs when the engine is under load can be caused by a faulty power valve inside the carburetor. A carburetor uses intake vacuum to pull fuel through its metering circuits. As engine load increases and the throttle opens wider, intake vacuum drops. This can reduce the flow of fuel and make the fuel mixture go lean, so the power valve has a spring-loaded vacuum-sensing diaphragm that opens to increase fuel flow when vacuum drops. If the diaphragm has failed or the valve is clogged with dirt or fuel varnish deposits, it must be replaced. A new power valve is usually included with a carburetor rebuild kit. Hesitation or misfiring under load can also be caused by a weak ignition coil, or cracks in the coil or distributor cap, or bad spark plug wires. Engine Stalling An engine can stall when cold if the fast idle speed is not set high enough. It may also stall when it has warmed up if the idle speed is set too low, if the idle the fuel mixture is too lean, if the fuel is contaminated with water (or too much alcohol), or if the if there is not enough fuel pressure to keep the carburetor bowl filled. Adjusting the fast idle, regular idle speeds and/or idle mixture adjustments can often eliminate a hot or cold stalling problem. The fast idle linkage increases idle speed when the engine is cold so it will not stall. Adjusting the choke for a richer setting may solve the problem. If the Idle Mixture adjustment screws are adjusted too lean, the engine may stall. Stalling can also be caused by air and vacuum leaks in the carburetor itself (leaky gaskets and seals) between the carburetor base plate and intake manifold (bad base gasket), or in any of the vacuum hoses that connect to the carburetor or intake manifold. If air is being sucked into the engine though a vacuum lea,k, it will lean out the Air/Fuel mixture causing a rough idle and stalling. The cure is to locate and repair the vacuum leak. Stalling can also be caused by a…

Fonte: AA1Car.com