Float type carburetors have one main disadvantage. They work perfectly well in horizontal position, however when carburetor tipped on its side, the float will no longer be useful for controlling fuel flow. Many devices powered by small engines need to be operated in different positions, a chain saw for example. Obviously a float type carburetor not suitable for a chain saw; so other types carburetor must be available.


To maintain constant fuel level when carburetor is out horizontal position, diaphragm type carburetor used. Diaphragm acts as fuel pump. The diaphragm type carburetor has two internal valves and a spring loaded diaphragm. Two valves act to check flow of fuel, allowing fuel to flow from tank to fuel bowl but not backwards.

Consider how typical diaphragm type carburetor operates as engine’s piston descending. Piston traveling down in cylinder creates partial vacuum at outlet side of carburetor. Vacuum will pull carburetor’s diaphragm upward against spring pressure. Valve 1 positioned in carburetor so that vacuum created by moving diaphragm draws valve away from its opening. Fuel can flow out of pickup tube, into space previously occupied by diaphragm. Fuel can move no further because vacuum created by moving diaphragm has pulled Valve 2 closed.

Once piston has reached bottom of its stroke, air no longer flows through carburetor, ending partial vacuum at diaphragm. Spring can now push diaphragm back down. Diaphragm returns to previous position, presses down on fuel. Pressure causes Valve 1 to close, shut off flow through fuel pickup tube. Meanwhile pressure from fuel under diaphragm pushes open Valve 2. Fuel allowed to empty into fuel bowl.

Certain carburetors use diaphragms for reasons other than pumping fuel. Sometimes diaphragm used to operate choke plate. Choke plate used to enrichen fuel mixture during starting. When engine being started, choke plate closed, providing richer air to fuel ratio. Once engine started, choke plate opens and carburetor functions to adjust ratio.

Consider typical carburetor that uses diaphragm to control choke plate. Has small metal rod that links diaphragm to choke plate. When engine isn’t running, no vacuum and spring will hold up diaphragm, keeping choke plate closed. Vacuum passage exists from intake manifold to lower part carburetor and under diaphragm. Once engine starts, vacuum pulls diaphragm down, opening choke plate.

This way operating choke plate has several advantages. Most important is operator doesn’t have to close and open choke manually. Choke controlled automatically as engine stops and starts. Additional advantage is that diaphragm controlled by engine vacuum. Not only will choke be closed during starting, it will close slightly if engine operated under heavy load and vacuum in engine is low. Richer fuel mixture that results will make engine running under heavy load perform better.


In type carburetor discussed previously, force required to pull fuel through main pickup tube provided by air passing through venturi. Venturi speeds up airflow, producing low pressure area. Pickup tube from fuel supply ends in venturi. Because of low pressure there, fuel drawn out pickup tube into engine. This takes place whenever air passing through venturi.

Are carburetors that operate on different principle. Such carburetor uses vacuum created by engine during intake stroke to pull fuel from fuel supply tube. Since this carburetor doesn’t rely on hourglass shaped venturi, and uses suction created by engine, commonly referred to as suction type carburetor.

Consider typical suction type carburetor. Normally doesn’t use a fuel bowl. Instead mounted directly on top fuel tank. Pickup tube extends from bottom carburetor into fuel tank. As vacuum created in carburetor, fuel drawn up pickup tube.

This type carburetor doesn’t rely on traditional venturi to produce low pressure or vacuum. Instead carburetor opening where air passes through somewhat smaller than opening on venturi style carburetor. Since airflow restricted as it passes through smaller opening, greater vacuum force forms inside suction type carburetor. This type carburetor usually have a restricting component on inlet side so airflow restricted even more.

Additional restricting component on inlet side in the form of small ring. This ring slightly narrows opening for short distance within inlet. Will always be some vacuum force in carburetor. In normal float type carburetor, very weak vacuum created when throttle wide open. With restricted inlet on suction type carburetor, stronger vacuum created.

Suction type carburetor typically uses throttle plate that’s larger than normal. Purpose to provide more restriction to air flowing through carburetor. Thicker plate will cause some restriction even when wide open. The more airflow restricted, the more vacuum or suction created inside carburetor. Normally ratio air to fuel must be kept within limits so engine will run efficiently. So airflow in engine must not be restricted too much. Flow should be restricted only enough to develop suction necessary to pull fuel out pickup tube into engine.

One disadvantage to suction type carburetor. Gets fuel directly from fuel tank. How easily fuel can be drawn into carburetor depends on amount in tank. When tank full, fuel can be drawn up easily. When tank full, air and fuel mixture comparatively rich. When fuel level low in tank, harder to draw into carburetor. Mixture then will be leaner.