Impulse Fuel Pumps; Float Style Carburetors; Diaphragm (Pressure Differential) Carburetors - Tecumseh ECV100 - 120 Technician's Handbook

IMPULSE FUEL PUMPS

Impulse fuel pumps may either be mounted externally onto
the carburetor fuel inlet or remotely mounted. These pumps
are connected in the fuel line between the fuel supply and
the carburetor or directly to the fuel inlet.
Impulse fuel pumps are operated by crankcase impulses
created by the up and down movement of the piston. A
hose called a pulse line connects the fuel pump diaphragm
chamber to the crankcase and transmits these impulses
to the pump diaphragm. The impulses actuate the
diaphragm and flap valves to lift the fuel from the fuel tank
to the carburetor (diag. 6).

FLOAT STYLE CARBURETORS

A float is used to maintain the operating volume of fuel in
the carburetor bowl. As the fuel is used by the engine, the
fuel volume in the carburetor bowl drops and the float moves
downward. This allows the inlet needle valve to move off
the sealing seat. Fuel flows by gravity or a pulse pump into
the fuel bowl. As the fuel volume in the bowl again rises, it
raises the float. This upward float motion moves the inlet
needle valve to the closed position. When the needle
contacts the seat, the fuel flow is stopped. The tapered
end of the inlet needle varies the fuel flow rate so that the
fuel volume in the carburetor bowl will remain constant (diag.
7). The float height is set according to the service procedure.
DIAPHRAGM (PRESSURE DIFFERENTIAL)
CARBURETORS
This type of carburetor uses a rubber-like diaphragm which
is exposed to intake manifold pressure on one side and to
atmospheric pressure on the other. Tecumseh diaphragm
carburetors use the diaphragm as a metering device. As
the intake manifold pressure decreases due to downward
piston travel, the atmospheric pressure on the vented side
of the diaphragm moves the diaphragm against the inlet
needle. The diaphragm movement overcomes the spring
tension on the inlet needle and moves the inlet needle off
the seat. This permits the fuel to flow through the inlet
valve to maintain the correct fuel volume in the fuel chamber.
The inlet needle return spring closes the inlet valve when
the pressure on the diaphragm equalizes or a pressure
higher than atmospheric exists on the intake side (upward
piston travel). The diaphragm meters a correct fuel volume
in the fuel chamber to be delivered to the mixing passages
and discharge ports (diag. 8).
A main or idle adjustment needle may be replaced by an
internally fixed jet on some models.
The main nozzle contains a ball check valve. The main
purpose of this ball check is to eliminate air being drawn
down the main nozzle during idle speeds and leaning the
idle mixture.
An advantage of the diaphragm carburetor over the float
system is that the diaphragm carburetor increases the angle
that the engine may be operated at.
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