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Understanding Carburetors

To operate efficiently the Air/Fuel Mixture must be 12 to 14 units of air per unit of fuel by weight.

Ratio of Air to Fuel

The ratio of air to fuel is based on weight - not volume - and varies with engine speed and operating conditions. An air/fuel mixture with too much fuel is described as being "too rich". Mixtures that are too rich waste fuel and they accelerate the formation of carbon deposits on pistons rings, combustion chambers, and exhaust ports. These deposits reduce performance and can even damage the engine.

Rich mixtures can also foul spark plugs, this is because fuel and oil accumulate on the spark plug´s insulator. Electricity flows through the deposits on the insulator´s surface, rather than jumping the gap between the plug´s electrodes. If deposits remain on the spark plug after the engine is shut off, it may be difficult to start.

Foul Spark Plug

When there is not enough fuel in the Air/Fuel mixture ratio, it is described as being "too lean". Mixtures that are too lean may burn too hot. On carbureted two-stroke engines, the lubricating oil is mixed with the fuel, so mixtures that are too lean don´t have as much oil as they should. A few seconds of high-speed operation with a mixture that is too lean can permanently damage the pistons and other internal parts.

Damaged Piston Due to Lean Mixture

The challenge is to provide the correct mixture for continuously changing load conditions and engine speeds.

All carburetors rely on atmospheric pressure, which is the weight of the air surrounding us. Air doesn't weigh very much, but there are miles of it above us. Although you can't feel it, about 15 PSI of atmospheric pressure is constantly pushing against you from all sides.

Atmospheric Pressure Around the Earth

Internal Combustion Engine Basics

When a two-stroke engine is cranking or running, the pistons create a low-pressure area inside the engine every time they move away from the crankcase.

Piston Creating Low Pressure

On Four-Stroke Engines, the Intake Stroke Creates a Low-Pressure Area.

Atmospheric pressure pushes air through the carburetor's throat to fill this low-pressure area. Carburetors use this principle not only to supply air to the engine but also to move fuel and mix it with the air.

Air pushing through the carburetor´s throat

Diagram Carburator Circuit

Float Circuit: Maintains a small supply of fuel in the float chamber.

Inner Workings

  1. The level of the fuel is critical because it affects the mixture ratio, so the float circuit must keep the fuel level constant. The float valve controls the fuel flowing into the float chamber.

  2. When the fuel level drops, the float drops with it, opening the float valve. Fuel pump pressure forces the fuel through the valve´s seat, past its needle, and into the float chamber.

  3. When the fuel level rises, the float rises with it, closing the valve and shutting off the fuel. This maintains the fuel supply at a constant level.

  4. To transfer the fuel from the float bowl to the carburetor´s circuits we will use atmospheric pressure to lift fuel and move it through the carburetor´s circuits.

A. The float chamber is vented, so the fuel in the chamber is under atmospheric pressure.
B. A tube or a nozzle, connects the float chamber to a passage in the top of the carburetor.
C. This passage exits into the low pressure area behind the throttle plate.
D. Atmospheric pressure forces fuel through the high-speed office, up the tube into the passage.
E. Through the calibration pocket.
F. Past the idle mixture needle.
G. Then into the carburetor throat.

The throttle bore has two small ports, just ahead of the closed throttle plate. These ports are also exposed to atmospheric pressure.

This causes air to enter and combine with the fuel in the calibration pocket. The resulting air and fuel mixture flows past the idle mixture needle and merges with the air flowing through a hole or slit in the throttle plate, creating a combustible mixture.

When the throttle plate opens far enough, it exposes the first intermediate fuel port to low pressure. Now, fuel begins to flow from the first port. Because only one intermediate port is venting air into the calibration pocket and idle circuit, a richer mixture flows from the first port and idle circuit into the throttle bore. This combines with the increased airflow past the throttle plate to create the correct ratio.
Opening the throttle plate more exposes the second intermediate fuel port to low pressure. Now fuel flows into the throttle bore from both ports and the idle circuit.

Intermidiate Port Mixture

Note: The intermediate circuits don't just deliver increased amounts of the same mixture; they deliver increased amounts of a richer mixture.

The intermediate fuel mixture is controlled by the size of the intermediate orifice. This orifice controls fuel - or air - depending on the design of the carburetor. If it is hidden inside or covered by a plug, it controls fuel. If it can be seen without removing a cover or plug, it controls air - and is often called an ¨air-bleed.¨ Increasing the size of an air-bleed orifice has the opposite effect of increasing the size of a fuel orifice. A larger air-bleed allows more air to flow into the calibration pocket and results in a leaner mixture. A larger fuel orifice allows more fuel to flow into the calibration pocket and results in a richer mixture.


Intermediate Orifice & Air Bleed Orifice respectively

** Important Concept **

When an air stream speeds up, its pressure decreases. The narrow section of the carburetor´s throat is called the venturi. As we open the throttle, more air flows through the venturi, and as such its speed increases. This causes the air pressure to drop in the venturi. Atmospheric pressure in the float chamber pushes fuel up the fuel nozzle toward the venturi.

As the throttle continues to open, low pressure created by the engine extends forward and over the high-speed fuel nozzle, causing fuel to rise higher in the nozzle. At the same time, more air is flowing through the venturi, so the air stream speeds up and the pressure drops in the venturi area. The high-speed fuel nozzle extends into this fast-moving air stream. It acts as an airfoil, creating a powerful low-pressure area on the engine side of the nozzle. When the pressure in this area is low enough, atmospheric pressure fuel goes through the high-speed orifice, up the high-speed nozzle, and into the venturi, where it mixes with air and creates a combustible mixture.

Low Pressure Venturi

Fuel Flow

When the throttle plate approaches wide open, atmospheric pressure forces more fuel through the nozzle well and into this low-pressure area and because the high-speed orifice limits the fuel flow, the fuel level in the nozzle well drops. This exposes bleed holes in the high-speed fuel nozzle.

Wide Open Throttle

The nozzle well is vented to atmospheric pressure, so air enters the bleed holes and mixes with the fuel, making it easier to lift and atomize. The size of the vent allows the correct amount of air to flow into the nozzle well.

So that is how the carburetor provides the proper mixture at all throttle settings, from idle to wide open.

Important Note

A damaged or missing nozzle well gasket will allow too much air to enter, and cause a lean mixture a high speeds. The ratio of air to fuel is controlled by the size of the high-speed fuel orifice.
A large orifice allows more fuel to flow into the high-speed nozzle well, resulting in a richer mixture, and a smaller orifice results in a leaner mixture.

Damaged Nozzle

Starting an Idled Engine

Now, when the engine is cold, it needs a richer mixture to start, and to run smoothly until it warms up. Most outboards use a primer for this purpose. The primer may inject fuel into the rear of the carburetor or in other models, it injects fuel into the intake manifold or crankcase.

But, some outboards don´t have a primer to assist in starting; they use a choke instead.
A choke plate is located in front of the venturi and it should be closed when starting a cold engine.
The choke plate doesn't cut off air completely; it has a hole or slot that lets some air enter the carburetor throat. When the engine cranks or runs, this restricts the airflow into the engine, creating a low-pressure area in the carburetor throat.
Then atmospheric pressure in the nozzle well pushes fuel up the high-speed nozzle, and into the carburetor throat and this provides the richer mixture that the engine needs to start.

Fuel Injection and Choke Respectively


Pozi-Drive screws are sometimes used in outboard carburetors and other applications. These screws require the use of a Pozidrive screwdriver because a Phillips screwdriver will damage the head of a Pozidrive screw.

Proper Screw Drivers

Orifice Drivers are designed to remove orifices safely; the driver´s tip and round shank prevent damage to the orifice, and to the threads in the carburetor. Always inspect orifices closely, look for foreign matter to its body.

Several different styles of orifices are used in carburetors.

Orifices with 1/4 - 20 threads are stamped with a C or D, indicating the direction of fuel flow because orifices are calibrated for a specific direction of fuel flow.
They have different flow characteristics when fuel flows in the opposite direction.

Installing a C orifice in place of a D orifice, or vice-versa, can drastically affect engine operation, and even cause serious powerhead damage. Use the parts catalog to verify the correct type and size of orifices.

Types of Orifices

To remove core plugs from the carburetor body, use a small hammer and punch with a 1/8 inch tip. Then, drive the punch through the center of the core plug. Use several light taps of the hammer; not one or two hard blows and use the punch to pry the core plug out of the carburetor.
Do not drive the punch more than 1/8 inch below the top of the calibration pocket!

Removing Core Plugs

Following that, inspect the ports in the calibration pocket and make sure all the holes are clear of obstructions.
Inspect all gasket surfaces for nicks or irregularities. The surface of the nozzle well should be flush or slightly below the surface for the fuel chamber gasket.

Inspect all gaskets for damage. An incomplete imprint can help you find problems with the sealing surfaces. Always replace gaskets; never reuse them.


The carburetor supplies the engine with the correct fuel/air mixture. To accommodate a variety
of operating speeds and loads, the carburetor must adjust the amount of the fuel/air mixture it
delivers to the engine.
Atmospheric pressure lifts the fuel and moves it through the carburetor’s circuits.
The carburetor consists of the following circuits:
• The float circuit – controls the fuel flow into the float chamber.
• The idle circuit – controls the fuel flow with the throttle plate closed.
• The intermediate circuits – control the fuel flow when the throttle plate begins to open.
• The part throttle circuit – controls the fuel flow when fuel begins to enter into the venturi
through the high-speed fuel nozzle.
• The high-speed circuit – controls the fuel flow through the high speed fuel
• The primer circuit – provides a richer fuel/air mixture for starting.

Test Your Knowledge

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Carburetors Quiz

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