Info For Carburetor Men

[mitsugdi]

Cold Starting Problem . . . . . . .
CONDITION POSSIBLE CAUSE CORRECTION
Engine cranks but will not start. 1) Choke not closing. 1) Inspect choke adjustment and for something binding. Adjust if necessary.
2) Choke linkage binding. 2) Lube with WD-40 and check for something bent. Adjust if necessary.
3) No gas in carb. 3) Check fuel delivery. Look for plugged filter or clogged lines, bad pump, stuck needle & seat, and fuel pressure.
4) Accelerator pump defective or blown out. 4) Replace the pump. Problem is usually caused by bad gas, dirt in gas, or vacuum leak or ignition problems causing engine spitback.
5) No spark or engine problems such as bad compression. 5) Diagnose & correct the problem.
Engine starts, then dies within a few seconds. 1) Choke not closing properly. 1) See notes above. Adjust choke if necessary.
2) Big vacuum leak on engine somewhere. 2) Use vacuum gauge to check. Fix the leak. You may have put the base gasket on wrong or it is the wrong one for this carb & engine combination.
3) Choke pull off setting incorrect. 3) Adjust to factory specs. Carb can get bumped in shipping or transportation and can accidentally change the setting.
4) Fast idle RPM set too slow. 4) Adjust to recommended RPM.
5)Low fuel delivery. 5) Correct delivery to carb. Usually it is a plugged up filter.
6) Electrical or compression problems on the engine. 6) Do complete tune up & diagnosis. Fix the problems found.
7) Float level set very low. 7) Check & adjust the float level to factory specs.
No electricity up to idle solenoid, or no ground. Check for power & ground. Correct the problem.
9) Defective idle solenoid. 9) Replace it.
Engine normally starts OK but then dies backing out the driveway or at the first stop sign. After that it runs OK. 1) Choke not set tight enough. 1) Set choke a little tighter & try it.
2) Choke pull-off opening too much. 2) The setting varies depending upon the exact application. One carb may fit several applications. Set the pull-off so the choke is a little tighter.
Engine starts OK, increases RPM then gets too slow with lots of black smoke. 1) Choke set too tight. 1) Adjust a little (1/8") looser.
2) Pull-off set too tight. 2) Adjust so it is open a little more.
3) Slow flooding. 3) Fix cause of flooding. (see "flooding" section coming soon)
4) Float level very high. 4) (Rare) Set to factory specs.
5) Power valve blown. (only happens on Ford & Holley) 5) Replace the power valve.
6) Pull-off diaphram blown. 6) Caused by installer allowing engine to spit-back up through carb. Replace the pull off.
7) (on Carter) Pump blown out 7) Same cause as #6 above. Replace the pump.
Same cause as #6 above. Replace the float.
9) Choke spring may be backwards and is getting tight when heating up instead of loose. 9) Remove choke cover, cool off choke, reverse spring, reinstall cover and set tension to factory specs.
Engine starts, then races for a few seconds and then dies every time. 1) Big vacuum leak somewhere. 1) Correct the vacuum leak. Make sure you haven't forgotten to hook up a hose somewhere. Base gasket may be wrong one or on wrong.





Warm Starting Problem . . . . . . .



CONDITION POSSIBLE CAUSE CORRECTION
Engine cranks but will not start. 1) Carburetor flooding. 1) See section on "flooding" for details on how to correct.
2) Choke is closed when engine is hot. 2) Find & fix cause for choke staying closed. Look for no heat source, spring in backwards, or something jammed or bent.
3) No spark. 3) Do complete tune up.
4) No compression. 4) Diagnose cause of no compression & fix.
5) No fuel 5) Check fuel delivery volume and pressure. Look for clogged lines, filter, or pump. Check for kinked or swollen fuel lines.
6) No air. 6) Check for clogged air filter, especially after driving through muddy or dusty area.
7) Too much air 7) Look for big vacuum leak, such as broken hose, blown gasket, bad power brake diaphram, bad PCV valve.
Engine starts, then dies within a few seconds. 1) Choke is staying closed 1) Diagnose and fix choke problem.
2) Flooding 2) See section on flooding.
3) Power valve blown. 3) Replace power valve.
4) Venting system failure. 4) Check out entire fuel system venting system, inluding the vent valve on the carb, the charcoal canister, all hoses & check valves in the system, and any solenoids that trigger the vent system to operate.
5) Idle jet plugged up with dirt. 5) Clean out the idle jet and any other dirt in the carb.
6) Idle air bleed plugged up or missing. 6) Check idle air bleed. Clean or replace as necessary.
7) Idle cut-off solenoid not working. 7) Check idle solenoid, especially for power to it and ground to it, replace solenoid if necessary.
Engine starts, then races for a few seconds and then dies. 1) Big vacuum leak somewhere. 1) Find the vacuum leak & fix it. Also look for wrong base gasket or one that is installed wrong. If engine has been spitting back, it may have blown out the base gasket or a gasket in the carb.
Engine starts OK, but then get real rough. Lots of black smoke. 1) Power valve blown out by spitback up through carb. 1) Replace the power valve.
2) Slow flooding. 2) See flooding section.
3) Float sunk. (usually caused by spitback up through carb.) 3) Replace float.
4) Venting system problem. 4) Check entire venting system & fix.





Cold Engine Drivability Problem . . . . . . .



CONDITION POSSIBLE CAUSE CORRECTION
Engine stalls when transmission is put into gear. 1) Incorrect choke pull-off adjustment. 1) Readjust the pull-off or replace if necessary.
2) Fast idle RPM incorrect (too slow) 2) Speed up the fast idle to factory specs.
3) Engine running too lean because of vacuum leak or dirty jet. 3) Check for vacuum leak. Flow test carb to check jetting.
Hesitation, stalling, stumbling, flatspot, or deadspot during acceleration: Backfiring or spitback up through carb. 1) Vacuum leak. 1) Check for vacuum leak & fix it.
2) Ignition timing retarded too far. 2) Reset timing.
3) Accelerator pump nozzle has dirt in it. 3) Clean out the nozzle tip.
4) Accelerator pump cup swollen up from contact with bad gas or chemicals. 4) Replace the pump cup.
5) Economizer jet too small or partly blocked. 5) Clean out economizer jet & check the size.
6) Choke pull-off open too far. 6) Adjust the pull-off tighter.
7) Secondary throttle plates not closing all the way. 7) Fix it.
Vacuum hoses hooked up wrong. Connect up right. Be especially careful of the EGR & Dist. connections: sometimes the pipe locations are reversed on Rochesters.
9) Idle jet partly blocked with dirt. 9) Clean out the jet & any other dirt that is in there.
10) Distributor timing not advancing properly. (worn breaker plate, worn shaft, pin hole in diaphram, crack in hose, etc.) 10) Check distributor and all related systems carefully. Replace defective parts.
Hesitation, deadspot or stalling after first mile of warmup. 1) Defective electric assist on choke 1) Replace it.
2) Defective accelerator pump (low output). 2) Replace it.
3) Float level setting very low. 3) Adjust to factory specs.
4) Bad ignition condenser. 4) Replace it.
Periodic backfiring with black exhaust smoke: Deadspot, flatspot, hesitation, stumbling, backfiring. 1) Plugged heat crossover system in manifold. 1) Inspect and clean passages in intake manifold and heads. Test heat riser valve, replace if defective.
2) Defective source of hot air up to the carburetor. 2) Check and replace as necessary: heat shroud duct, temperature sensor, vacuum door motor, manifold vacuum supply.





Warm Engine Drivability Problem . . . . . . .



CONDITION POSSIBLE CAUSE CORRECTION
Hesitation under light throttle: Deadspot & stumble. 1) Vacuum leak somewhere, or hose off or hooked to wrong vacuum fitting. 1) Inspect hoses. Route and lead the hoses correctly. Look for leak because of wrong base gasket or it was installed upside down.
2) Accelerator pump problems. 2) Inspect and adjust pump stroke, pump plunger, discharge nozzles and check valves. Inspect the accelerator pump, look for swollen pump cup.
3) Float level set very low. 3) Set float to factory specs.
4) Ignition timing retarded. 4) Set to factory specs. Make sure advancing correctly.
5) Dirty idle jet or economizer jet. 5) Inspect idle jets. Clean as necessary.
6) Idle speed set too fast & mixture is too lean (common!). 6) Richen up the idle mixture, reset idle speed to factory specs, then lastly reset the mixture using the lean drop method.
7) Idle cut-off solenoid not working, or no power to it or no ground to it. 7) Inspect & fix as necessary.
Frozen or binding heated air inlet (stuck in full hot or full cold position). Inspect & fix as necessary.
9) EGR valve stuck on or coming on too early (hose on wrong?) 9) Inspect hose routing to EGR valve & inspect valve. Replace as necessary.
Doggy, runs rough, lots of black smoke at idle. 1) Choke staying closed or partly closed. 1) Fix choke or heat source.
2) Slow flooding. 2) See flooding section.
3) Power valve blown (caused by engine spitting back.) 3) Replace power valve.
4) No electricity or heat source to choke. 4) Fix cause of no heat source or electricity to choke.
Hesitation under heavy throttle: Deadspot & stumbles. May backfire or spitback. 1) Defective accelerator pump. 1) Look for dirt in pump nozzles, swollen cup from bad gas, or check ball missing or stuck.
2) Metering rods or power valve sticking or binding. 2) Inspect and correct.
3) Vacuum leak. 3) Locate leak and correct.
4) Float level setting very low. 4) Reset to factory specs.
5) Plugged up fuel filter, defective fuel pump, or swollen or kinked lines. 5) Inspect and replace parts as necessary.
6) Secondary air valve set wrong. 6) Check & adjust the secondary air valve spring.
7) Ignition timing retarded. 7) Set to factory specs. Check for proper advancing.
Dies coming up to a stop sign, but idles ok. 1) Bad or misadjusted BCDD (if equipped). 1) Adjust to specs and replace it.
2) Bad throttle positioner or bad vacuum source to it. 2) Check throttle positioner with a vacuum pump. Replace if defective. Replace any cracked hoses. Make certain that the vacuum hose is connected to the correct pipe on carb or on the thermal switch. Make sure all related pipes have vacuum.
3) Idle speed and mixture incorrectly adjusted. 3) Reset to specs using Recarbco's method. See the adjustment and installation instructions on our website.
4) Loose or defective float pin. 4) Fix or replace the pin.





Poor Performance or Gas Mileage . . . . . . .



CONDITION POSSIBLE CAUSE CORRECTION
No power or bad gas mileage. 1) Plugged exhaust. 1) Check exhaust. Look for plugged catalytic converter, bad muffler baffle, kinked or crimped pipe, dirt or other foreign matter in pipe.
2) Clogged gas tank vent, or fuel venting system. 2) Remove gas cap & see if performance improves. If so clean or replace the gas cap. Check the charcoal cannister, hoses to it & any check valves. Check the electric vent valve on the carb if there is one, & make sure it is getting power at the right time.
3) Ignition timing retarded 3) Check timing at idle and also for full advance when revved up. Look for bad or loose hose, hose hooked up to wrong pipe on distributor, bad thermal switch, leaking vacuum advance can on the distributor, a worn breaker plate, worn distributor shaft, sticky weights, point gap closed up, etc.
4) Clogged air filter 4) Replace air filter.
5) Choke not opening 5) Fix choke or heat source problem.
6) Secondary not opening. 6) Check the lockout: secondaries won't open unless choke is coming off all the way. Check for sticky or bent shaft or linkage. If air valve type (Rochester), check the spring tension, the plastic cam, & metering rods for dragging or sticking.
7) Wrong main jets or rods 7) Check them. Replace if necessary.
Dragging brakes. Fix brake problem.
9) Low tire pressure. 9) Increase tire pressure at least to factory recommendations.
10) Automatic transmission malfunction. 10) Diagnose & fix automatic transmission.
11) Wrong or malfunctioning thermostat in cooling system. 11) Replace thermostat.
12) Blocked or leaking exhaust heat passage in intake manifold. 12) Remove manifold & fix it.
13) Defective accessory (power steering pump, air conditioning compressor, etc.) causing drag on engine. 13) Diagnose & repair or replace the accessory unit.
14) Wheels out of alignment. 14) Have wheels professionally aligned.
15) Poor driving habits. 15) Reduce speed, quick acceleration, screeching around corners etc.
16) Float level much too high, or float partly sunk. 16) Set to factory recommendations, & check float weight, replace if necessary.
17) Ignition problems or needs a tune up. 17) Do complete tune up & physically inspect the cap, rotor, wires, coil, plugs, points, etc. in addition to checking on the scope.





Flooding . . . . . . .

(Note: flooding is gas pouring out uncontrollably, it is NOT hesitation!)

CONDITION POSSIBLE CAUSE CORRECTION
Gas pours out when engine is turned off. 1) Charcoal cannister is saturated with gas. 1) Replace the charcoal cannister.
2) Vent valve is not functioning correctly. 2) Replace the defective valve or fix other cause of it's malfunctioning (like loose connection, etc).
3) Kinked hoses in venting system. 3) Replace the kinked hoses with correct kind.
4) Stuck or blocked check valves in the vent hoses or elsewhere in the system. 4) Replace the check valves.
5) Gas tank vent is blocked. 5) Fix it or replace the gas cap if vent is in the cap.
6) Gas line located too close to a heat source (such as a radiator hose or exhaust manifold). 6) This causes the fuel to expand & be forced past the needle & seat. Also, fuel can boil in the carb if there are improper or missing gaskets or spacers between the carb & manifold. A heat riser stuck in the closed position will also cause boiling and flooding.
Gas pours out when the engine is running.

Note 1: often removing the carb from the manifold & shaking it hard a couple of times will effect a cure.

Note 2: if procedure in note 1 doesn't cure it, the problems is almost always dirt or bad gas getting into the carb. Carb will then need to be disassembled to fix it. 1) Dirt stuck on tip of the needle & seat. 1) Clean the tip off carefully or replace the needle & seat and clean the entire fuel system out.
2) Idle speed is set too fast. 2) Reduce the idle speed to factory specs. If it will not idle slowly, the mixture is set too lean, or the idle solenoid is not functioning right, or there is a vacuum leak somewhere (usually not in the carb).
3) Idle solenoid is not functioning. 3) Check for power & ground up to the solenoid. Check the operation of the solenoid. Look for dirt stuck in the solenoid or the passageways to it.
4) Fuel pump pressure too high. 4) Check pressure. Use a regulator to control the pressure or put on a new pump (don't use high pressure type).
5) Fuel pressure is fluctuating or "pulsing" (common on Colts, Mitsubishi & other Chrysler imports). 5) Control pulsing with a regulator.
6) Float is heavy or sunk. 6) Replace float. Is often caused by spit-back up through carb.
7) Gas is very gummy, old, or has too much alcohol or other chemicals in it. 7) Clean bad gas out of carb. Clean out of fuel system. Use only good quality gas.
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9) Choke not working. 9) Check cause of choke failure, choke pull-off failure, or loss of heat to the choke.
10) Needle not seated properly during initial fill. 10) Try tapping the fuel inlet or needle & seat area with the handle of a screwdriver. If this doesn't work, unbolt carb from manifold and shake it hard a couple of times.
11) On Mikuni carb, the overturn ball fell out of place. 11) This is caused by the carb being overturned or turned on its side. Remove top and reinstall the ball.
12) On carbs with externally adjustable float levels. 12) Excessive adjustments have caused the seal to break. Replace the seals.





Hesitation During Acceleration . . . . . . .



CONDITION POSSIBLE CAUSE CORRECTION
Engine stalls when transmission is put into gear. 1) Incorrect choke pull-off adjustment. 1) Readjust the pull-off or replace if necessary.
2) Fast idle RPM incorrect (too slow) 2) Speed up the fast idle to factory specs.
3) Engine running too lean because of vacuum leak or dirty jet. 3) Check for vacuum leak. Flow test carb to check jetting.
Hesitation, stalling, stumbling, flatspot, or deadspot during acceleration: Backfiring or spitback up through carb. 1) Vacuum leak. 1) Check for vacuum leak & fix it.
2) Ignition timing retarded too far. 2) Reset timing.
3) Accelerator pump nozzle has dirt in it. 3) Clean out the nozzle tip.
4) Accelerator pump cup swollen up from contact with bad gas or chemicals. 4) Replace the pump cup.
5) Economizer jet too small or partly blocked. 5) Clean out economizer jet & check the size.
6) Choke pull-off open too far. 6) Adjust the pull-off tighter.
7) Secondary throttle plates not closing all the way. 7) Fix it.
Be especially careful of the EGR & Dist. connections: sometimes the pipe locations are reversed on Rochesters.
9) Idle jet partly blocked with dirt. 9) Clean out the jet & any other dirt that is in there.
10) Distributor timing not advancing properly. (worn breaker plate, worn shaft, pin hole in diaphram, crack in hose, etc.) 10) Check distributor and all related systems carefully. Replace defective parts.
Hesitation, deadspot or stalling that only seems to occur after the first mile of warmup. 1) Defective electric assist on choke 1) Replace it.
2) Defective accelerator pump (low output). 2) Replace it.
3) Float level setting very low. 3) Adjust to factory specs.
4) Bad ignition condenser. 4) Replace it.
5) Plugged heat crossever system in manifold. 5) Inspect and clean passages in the intake manifold and heads. Test heat riser valve, replace if defective.
6) Defective source of hot air up to the carburetor. 6) Check and replace as necessary: heat shroud duct, temperature sensor, vacuum door moter, manifold vacuum supply.
7) Defective thermal switch. 7) Check all thermal switches, especially those connected with the distributor, EGR, & vacuum motors in the air cleaner housing

SECTION TWO

Cross Sectional schematic of a CarburetorA carburetor basically consists of an open pipe, a "throat" or "barrel" through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi — it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

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Idle circuit
When the throttle valve is closed or nearly closed, the carburetor's idle circuit is in operation. The closed throttle reduces the airflow through the venturi to a level which cannot overcome the resistance to flow of the fuel, but it also means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel through small openings placed after the butterfly valve (and in SU and similar sidedraft carburetors to pull the piston and metering rod up).

Only a fairly small amount of air and fuel can pass through in this manner. Since this small volume of fuel/air mixture can generate so little force to keep the engine turning, keeping it running at idle is more difficult than keeping it running at higher speeds. Since the airflow is too low for the carburetor to respond at all, it cannot compensate for fluctuations;instead, idle airflow is set manually by the technician or mechanic, adjusting a screw which opens the throttle a tiny fraction to allow a minimal amount of air to pass, and another screw which serves as a valve in the idle fuel circuit to adjust the volume of fuel delivered. These adjustments interact with each other, as well as affecting manifold vacuum which affects distributor spark advance which in turn affects idle speed, so that adjusting the idle to optimum (highest manifold vacuum at the specified engine idle speed) is not a completely trivial operation. While experts often claim the ability to set the idle perfectly by ear, most individuals do a better job using a tachometer and vacuum gauge. Since the advent of emissions controls on production automobiles, the idle fuel flow is typically set at the factory on the "lean" side of optimal, by restricting fuel flow so that idle speed falls by 100 — 150 rpm from where it was when optimally adjusted, in order to reduce unburned hydrocarbons and carbon monoxide with some slight loss in reliable and smooth idling; the idle fuel adjustment is typically sealed at the factory to prevent tampering, so that adjustment when age and wear cause a large deviation from proper operation requires drilling out a plug over the adjusting screw or some similar modification to gain access.

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Off-idle circuit
As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes slightly higher in the carburetor throat; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

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Main open-throttle circuit
As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e. as the velocity increases, pressure falls). The venturi (sometimes a second or "booster" venturi is placed inside the venturi shaped into the carburetor throat to increase the effect) raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle located in the center of the venturi.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

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Power valve
For open throttle operation a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, engine vacuum decreases and the spring opens the valve to let more fuel into the main circuit.

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Accelerator pump
Similarly, the greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, causing a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump (often just a simple plunger which pushes down through a small tube filled with gasoline which feeds into the carburetor throat) which injects an additional amount of fuel as the throttle is opened to cover this lean period; this is usually adjustable for both volume and duration by some means, sometimes just bending the linkage. Often the seals around the moving piston parts of the pump wear out, so that pump output is reduced; this loss of accelerator pump action causes the characteristic stumbling or bogging under acceleration often seen in old, well worn engines until the seals on the pump are replaced. Specialized aftermarket kits are widely available for this purpose. Other variations of pump also exist, such as diaphragm based pumps.

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Choke
When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls fuel through the venturi to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke is connected to a "fast idle cam" or other such device which prevents the throttle from closing fully, which could starve the venturis of vacuum and cause the engine to stall. This also serves as a way to help the engine warm up quickly by idling it at a higher than normal speed. In addition, it increases airflow throughout the intake system which helps to better atomize the cold fuel and smooth out the idle.

In older carbureted cars, the choke was controlled by a cable connected to a pull-knob on the dashboard (GB — facia) operated by the driver. In most carbureted cars produced from the mid 1960s onward (mid 1950s in the United States) it is usually automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.

Some carburetors do not have a choke but instead use a mixture enrichment circuit, or an enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.

Classic British motorcycles, with side-draft slide throttle carburetors, used another type of "cold start device", called a "tickler". This is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. If the "tickler" was held down too long it also flooded the outside of the carburetor and the crankcase below, and caused a few fires in the process.

The main idea behind these devices is that extra fuel (a rich condition) is necessary to get a "cold" engine started and running for a short period of time. Either the air is restricted (choke), or more fuel is added (enrichener and tickler).

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Other elements
The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization.

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Fuel supply
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Float chamber
To ensure a ready supply of fuel, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in toilet tanks. As fuel is used up, the float drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and closes the inlet valve. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical adjustment, and the proper adjustment is indicated by lines scribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run well unless the float is replaced. The valve itself becomes worn on its sides by its motion in its "seat" and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial fuel stabilizer additives are available that reduce this problem.

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.

If the engine must be operated in any orientation (for example a chain saw), a float chamber cannot work. Instead, a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.

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Multiple carburetor barrels

Colombo Type 125 "Testa Rossa" engine in a 1961 Ferrari 250TR Spyder with 6 Weber 2-barrel carburetors intaking air through 12 "trumpets" visible on top of the engine, one individually adjustable barrel for each cylinder; the ultimate in tunability.While low performance carburetors may have only one barrel, most carburetors have more than one venturi, or "barrel", most commonly a two barrel, with 4 barrels being common in higher performance larger displacement engines, to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure "signal" from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. Similarly, in the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.

Similarly, multiple carburetors can be mounted on a single engine, often with progressive linkages; three two barrel carburetors were frequently seen on high performance American V8s, and multiple four barrel carburetors are often now seen on very high performance engines.

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Carburetor adjustment
Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too "lean". The "mixture" is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to petrol ratio is 14.6:1, meaning that for each weight unit of petrol, 14.6 units of air will be burned. In theory this the most efficient regarding the power/consumption ratio.

But as combustion chambers in use on engines are not able to allow complete combustion of the gasoline with stoichiometric mixture, a mix around 18:1 will give better results regarding fuel economy and pollution, the excess air allowing more complete combustion.

A richer mixture around 11:1 will deliver more power as the excess fuel will cool the cylinders and pistons but the price to pay is increased consumption and environmental pollution.

Carburetor adjustment can be checked by measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust gases.

The mixture can also be judged by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs.

In the early 1980s, many American-market vehicles used special "feedback" carburetors that could change the base mixture in response to signals from an exhaust gas Oxygen sensor. These were mainly used to save costs (since they worked well enough to meet 1980s emissions requirements and were based on existing carburetor designs), but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.

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History and development
The carburetor was invented by the Hungarian engineer Donát Bánki in 1893. Frederick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 successfully incorporating the carburetor as an important step forward in automotive engineering.

The word carburetor comes from the French carbure, meaning 'carbide' [1]. To carburete means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.

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Catalytic carburetors
There are persistent rumours that appear to extend into the realm of urban legend or even into conspiracy theory of extremely efficient carburetors. However, there may be some basis for these rumors or claims.

A catalytic carburetor mixes fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. The fuel would break down into methane, alcohols, and other lighter-weight fuels. The original catalytic carburetor was introduced to permit farmers to run tractors from modified and enriched kerosene.

The U.S. Army used catalytic carburetors in World War II in the North African desert campaign, it has been said, to achieve substantial logistic surprise and thus tactical and strategic advantage against the Germans.

However, it is known that less than two years after commercial introduction of the first catalytic carburetor, in 1932, tetraethyl lead was introduced as an additive to raise gasoline's resistance to spontaneous combustion, thereby permitting the use of higher compression ratios. Also in that time, the price differential between a thermal calorie of gasoline and kerosene was ended. Tetraethyl lead had the effect of poisoning catalytic carburetors. Many modern gasolines appear to have additives for "cleaning" which perform the same effect by producing varnishes or gums in the presence of water, which of course, is not a recommended use.

Gasoline/petrol is an impure mixture of linear heptane and octane and other miscellaneous light alkanes. Commercial gasolines usually contain additives to clean engines, artificially lower evaporation points, and (conjecturally) poison catalytic carburetors (an effect that is certainly real, but might be accidental).

Famed NASCAR mechanic Smokey Yunick spent many years working on a high fuel economy "vapor carburetor". The detailed operation is not widely disseminated, but the general principle is to heat the fuel with waste engine heat to enhance vaporization and improve the fuel's combustion characteristics. This was reasonably effective compared to normal carburetors of the time, but had implementation difficulties. During the years he was developing it, the average production engine moved from centrally located carburetors to electronic fuel injection, wherein the fuel is delivered right to the intake port. This dramatically reduces fuel condensation and puddling in the intake manifold and runners, making Smokey's design a solution to a problem which no longer existed so it was never commercially developed.

[sean]

good info

[cressida_override]

thas plenty info there......but looks good

[GSRturbo]

yes dread that is some killer info

*prints and save in archives*

[rych_bess2k6]

any suggestions here... if i try to pull off real fast or if i floor my gas pedal from stop my car rolls and sounds like it'd diein till it passes about 2300rpm then it pulls off. sometimes if i floor the gas th rpm will just drop and th car will roll a bit stutter and shut off... it's a ck1 lancer with th smallest 1.3 engine carb around. i think this 1.3 is smaller than usual. cause it performs like a .3litre engine pulling a truck. and i already got th suggestion with the gas and the matches.. i think th car may burn and the engine would survive just to torture me.

[fuzz_174]

very good info..

it deserves a bump

[Stay Tuned]

[centralized(m)]

KUDOS........SEARCHING FOR INFO ON THIS ALL MY LIFE THANKS MAN!!!

[cressida_override]

great info, thanks!

[centralized(m)]

please inform us of the link to this information