Carburetor

A carburetor blends air and fuel for an internal combustion engine.

Carburetor also spelled carbutator, carburettor or carburetter

Carburetors have largely been supplanted in the automotive industry by fuel injection.

The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

A remarkably tiny amount of gasoline is needed during each combustion cycle. Something on the order of 10 milligrams of gasoline per combustion stroke is all it takes!

The goal of a carburetor is to mix just the right amount of gasoline with air so that the engine runs properly. If there is not enough fuel mixed with the air, the engine "runs lean" and either will not run or potentially damages the engine. If there is too much fuel mixed with the air, the engine "runs rich" and either will not run (it floods), runs very smoky, runs poorly (bogs down, stalls easily), or at the very least wastes fuel. The carb is in charge of getting the mixture just right.

"Diagram of a basic carburetor"

If you look at a carburetor, you will notice a rather large hole going from one side to the other. This is called a Venturi. Air passes into the engine through this hole (Venturi). As the velocity of the air entering the carb (and then the engine) increases, it's pressure decreases, creating a low pressure or vacuum in the venturi. This vacuum moves around in the venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the venturi.

The way the jets are made causes the fuel to vaporize as it goes into the venturi. Where the jets are placed in the carb and where the jet's outlet is located in the venturi, determines what part of the throttle opening that jet controls. The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening. The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%.

This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet. As you can see, the different jets over lap the operating range of each other. That is, the jet needle starts to effect things before the effect of the idle jet ends.

This is something to remember when working on carbs... everything is interconnected. Change one thing and it will effect other things.

Three Carburetor Problems
1) Rich Mixture means the carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are:

• Poor fuel economy

• Sluggish acceleration

• Choke not needed from cold starts

• Sooty or black spark plugs

• Sooty or black muffler end pipes

• Strong smell of gasoline when machine is at idle

• Uneven running (will often slow from regular idle rpm's and stop)

2) Lean Mixtures means the carburetor is delivering too much air. Typical symptoms of a lean mixture are:

• Backfires as the throttle is closed (primarily during coast-downs)

• Lurching acceleration

• White or light grey spark plugs

• Requires excessive amounts of choke to run/start

• White or light grey muffler end pipes

• Bluing (on chrome systems) of the exhaust header down-pipes

3) Incorrect Adjustment applies to carburetors that have incorrect adjustment of the air/fuel screw and the balance between two or more carburetors - where fitted. Incorrect adjustment can produce any of the previously noted symptoms. On multi-cylinder machines, with separate carburetors for each cylinder, the following symptoms are typical of an adjustment problem:

• Poor overall performance

• Rattling sounds from the clutch

• Engine tends to stall easily

• Erratic acceleration

• Poor fuel economy

• Misfires and/or backfires

Correcting Carburetor Problems
Lean Mixtures: This condition is generally caused by the owner fitting after-market accessories such as exhaust systems, air filter systems or replacement carburetors of a different type or size. In addition, if the fuel level in the float chamber is set too low, insufficient fuel will be drawn through the main jet. Some carburetors have a slow speed fuel adjusting screw that regulates the fuel/air mixture in the lower rpm range.

The carburetor shown in the accompanying photograph has a low speed air adjusting screw. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will therefore richen the mixture (refer to a shop manual for correct settings).

If no changes have been made to the bike, and it previously ran well, a lean mixture can be traced to a leaking inlet manifold or leaking exhaust (often at the interface of header pipe and cylinder head).

Rich Mixtures: This condition is primarily caused by dirty air filters, but it could also result from the owner fitting replacement exhausts and/or carburetor systems. If the fuel level is set too high in the float chamber, a rich mixture will result.

Incorrect Carburetor Adjustment: This situation is mostly caused by poor maintenance. With the inherent vibration of all engines, carburetor parts (primarily adjusting screws) tend to rotate, and therefore change their positions. Low-speed running jets and multi-cylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.

Sources  Classic Motorcycles