SYSTEMS AND OPERATIONFUEL PUMP SYSTEMX The fuel pump on a diaphragm carburetor uses the vacuum and pressure pulse from the engines

crankcase to move a diaphragm up and down. The pulse travels from the crankcase through a drilled
passage or a hose to the carburetor.
Y As the diaphragm moves up it expands the area in the pump chamber. This creates a vacuum, whichdraws fuel through a one-way inlet check valve into the chamber.
Z As the diaphragm moves down and compresses the area in the pump chamber the fuel is forced throughthe one-way outlet check valve to the inlet needle.
It is important to make sure the passage from the engine crankcase to the pump diaphragm is clear andnot restricted in any way in order for the pump to operate properly.

FUEL METERING SYSTEM
The purpose of the metering system is to control fuel flow. It's operation is much like a float and inlet
needle on a conventional float bowl carburetor. The main advantage of this system is that it is not affectedby the orientation of the engine. The components of the fuel metering system consist of an inlet needle,
metering lever, metering spring and metering diaphragm.

X The pressure of the metering spring against the metering lever holds the inlet needle against its seat andprevents fuel from entering the metering chamber.
Y The metering diaphragm is made of a flexible convoluted material to allow for greater movement. As theengine runs, fuel is being drawn from the metering chamber in the carburetor. This causes the metering
diaphragm to move up and contact the metering lever. The pressure of the metering diaphragm against
the lever over rides the spring pressure on the inlet needle. The fuel pressure from the pump is then great
enough to overcome the spring pressure on the inlet needle and fuel flows into the metering chamber.

VENTURI PRINCIPLE OF OPERATIONX A venturi is a constriction of the inside diameter ofa tube. The “venturi affect” occurs when airflows
through a constriction, the airflow speeds up(velocity increases) and the pressure decreases(vacuum increases) at the point of the smallest
diameter.
The Venturi is used to produce low pressure inside
the bore of the carburetor. The low pressure pulls
fuel into the bore from the metering chamberwhere it mixes with the air and is drawn into thecombustion chamber.

PRIMER OPERATION
All the primer methods work basically the same.
X Under the primer bulb are an inlet and an outlet
check valve.
Y When the bulb is depressed the outlet check valve
is forced open and the air and fuel in the bulb
passes through the outlet check valve and into thefuel tank.
Z As the bulb is released and returns to it’s original
shape the outlet valve closes and a vacuum iscreated. The vacuum from the expanding bulb
draws fuel through the inlet check valve frominside the metering chamber.
[ The vacuum in the metering chamber draws in on
the metering diaphragm, and lifts the inlet needle
off its seat.
The
open
inlet
needlecausesthevacuum
createdby
theprimer
to draw fuel from the tank, through the\pump, into the metering chamber and up to the primer bulb.
] To prevent air from entering the metering chamber during primer operation there is a one-way check valve in
the idle circuit and main nozzle. Every time the primer bulb is depressed this process is repeated causing
any old fuel or air that was in the carburetor to be displaced by fresh fuel from the tank. The fresh fuel in thecarburetor makes the engine easier to start.
Note that once the metering chamber has been filled with fresh fuel continuing to push on the primer bulb will not
help improve the starting of the engine.

STARTING (CHOKE) OPERATION
X To start a cold engine the fuel and air mixture

entering the engine must be rich. Closing or
choking the air inlet of the carburetor using achoke valve accomplishes this. The choke valve
allows very little air to enter, causing the engine
vacuum to be concentrated inside the venturi ofthe carburetor.
Y The high vacuum draws fuel from the high and low
speed fuel circuits creating the very rich fuel andair mixture that is needed to start a cold engine.

The full-choke operation is used to get the engine tofire and start. In most cases the fuel air mixture is too
rich to allow the engine to run for a long time before it
dies. After the engine starts or “pops” and dies in thefull choke position the choke valve is then moved to thehalf-choke position. The engine is re-started and willcontinue to run in a rich condition.
After the engine has sufficiently warmed up in the half choke-position the choke valve can be moved to the fullopen position.

IDLE OPERATIONX At idle, fuel is delivered to the engine throughthe idle or low speed circuit. This circuit
usually consist of two or three holes locatedalong the throttle bore of the carburetor.

Y The throttle valve is slightly open allowing asmall amount of air to pass through to theengine. This creates a low pressure(vacuum) on the engine side of the throttle
valve and atmospheric pressure on the inlet
other side of the valve.
Z Air enters the secondary holes on theatmospheric side of the throttle valve andmixes with the fuel in the idle pocket creating
an emulsified air fuel mixture. The lowpressure draws the fuel and air mixture
through the first idle hole into the engine.

[ As the fuel and air mixture is drawn out of idle pocket it creates low pressure in the metering chamber.
\Atmospheric pressure pushes the metering diaphragm against the metering lever. This releases the springpressure applied to the inlet needle and allows fuel to flow into the metering chamber.
The main nozzle check valve is another important component of the idle circuit. This one way check valve
located in the main nozzle is under atmospheric pressure at idle. Without it air would flow from the main
nozzle into the metering chamber. This will cause an air leak in the metering chamber and there would be
no drop in pressure. The diaphragm will not move and the engine will die lean when idling. This check valve
also serves the same function during the priming operation.
X During part throttle operation or acceleration thePART THROTTLE OPERATION
throttle valve is opened to allow more air to theengine.
Y As airflow increases so must fuel flow tomaintain a proper air/fuel mixture. Thesecondary idle holes are now on the vacuumside of the throttle valve and fuel is drawn
through all the idle circuit holes.
Z The increased airflow causes vacuum to be
applied to the main nozzle. This allows fuelpressure from the metering chamber toovercome atmospheric pressure on thecheck valve and it begins to flow fuel to theengine.

WIDE-OPEN THROTTLE OPERATIONX The throttle valve is opened completely allowing
maximum airflow through the venturi.
Y The pressure drop from the venturi increases fuelflow from the main nozzle. Fuel from the main
nozzle and idle circuit mixes with the air, entering
the combustion chamber and satisfying therequirements of the engine.