Fuel Primers - Tecumseh TM SERIES Technician's Handbook

At IDLE the throttle shutter is almost closed, the low
pressure acts only on the primary idle fuel discharge port
due to throttle plate position. A relatively small quantity of
fuel is needed to operate the engine (diag. 3-3).
During INTERMEDIATE throttle operation, the secondary
idle fuel discharge port supplies fuel after it is uncovered
by the throttle plate. As the throttle plate opens
progressively further, engine speed increases. The
velocity of air going through the carburetor venturi creates
a low pressure area to develop at the main fuel discharge
port while diminishing the effect of the low pressure area
on the engine side of the throttle plate. When the pressure
at the venturi throat is less than that existing within the fuel
chamber, fuel is forced through the high speed mixture
orifice and out the main fuel discharge port (diag. 3-4).
At HIGH SPEED operation, the throttle shutter is in a full
open position. The air velocity through the venturi
increases which further lowers the air pressure at the
main fuel discharge port. All discharge ports are supplying
fuel as the adjustment orifices will allow (diag. 3-5).
The fuel pump diaphragm in the carburetor moves up and
down by pressure changes (pulsations) caused by piston
movement. The pulsations are transferred to the pump by
a passage called the impulse channel. The pump
diaphragm moves up drawing fuel into the pump fuel
chamber during a positive pulse, and a negative pulse
moves the diaphragm down forcing fuel out of the fuel
chamber through the inlet needle into the metering
chamber.

FUEL PRIMERS

Primers used on TC/TM engines supply a solid fuel
charge to the carburetor main nozzle (diag. 3-6). Fuel is
forced directly into the carburetor venturi. A choke shaft
and shutter is not needed or used when a primer is used.
This charge of fuel provides the rich mixture necessary to
start the engine.
Below the main nozzle is a one way check valve to
prevent fuel from being drawn back into the fuel chamber
when the primer bulb is released. The check valve also
prevents air from entering the carburetor during normal
engine operation.
12
Fuel
Impulse
Intake Air
Engine Vacuum
Idle
Fuel
Impulse
Intake Air
Engine Vacuum
Intermediate
Fuel
Impulse
Intake Air
Engine Vacuum
High Speed
3-3
3-4
3-5
3-6