Chapter 3 Carburetors And Fuel Systems; General Information; Operation - Tecumseh TC 200 Technician's Handbook

CHAPTER 3 CARBURETORS AND FUEL SYSTEMS

GENERAL INFORMATION

TC engines almost exclusively use diaphragm-type carburetors to be able to run effectively at any operating angle. The
diaphragm carburetors are produced by Walbro and Tillotson for Tecumseh. The carburetors use an internal diaphragm fuel
pump to supply the fuel to the carburetor fuel metering chamber. The metering diaphragm has one side exposed to
intake manifold pressure and one side exposed to atmospheric pressure. This diaphragm provides the same basic function
(maintaining the proper fuel level in the carburetor) as the float.
A limited number of TC engines were produced as outboards using a Tecumseh Series II float style carburetor. Consult the
Two Cycle Technician's Handbook (part # 692508) if service is required on this series of carburetor.
When servicing carburetors, use the engine model and specification number to obtain the correct carburetor part number. An
alternate method to find the correct carburetor part number on float type carburetors is to use the manufacturing
number and date code stamped on the carburetor and convert this number to a part number. In the carburetor section of the
Master Parts Manual or Microfiche Catalog, a cross reference chart will convert a carburetor manufacturing number to a
Tecumseh part number.

OPERATION

In the "CHOKE" or "START" position, the choke shutter is
closed, and the only air entering the engine flows through
openings around the choke shutter. As the recoil assembly
is operated to start the engine, downward piston travel
creates a low pressure area in the engine cylinder above the
piston. Higher pressure atmospheric air rushes into
the engine to fill the created low pressure area. Since the
majority of the air passage is blocked by the choke shutter, a
relatively small quantity of air enters the carburetor at
increased speed. The main nozzle and both idle fuel
discharge ports are supplying fuel due to the low air
pressure in the intake of the engine and the fuel side of
the main diaphragm. Atmospheric air pressure on the
opposite side of the main diaphragm forces the diaphragm
upward, depressing the inlet control lever, overcoming inlet
spring pressure and allowing fuel to enter the fuel chamber
through the inlet valve. A maximum fuel flow through the
carburetor orifices combined with the reduced quantity of
air that passes through the carburetor, make a very rich fuel
mixture which is needed to start a cold engine
(diag. 3-1).
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. 3-2).
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. 3-3).
6
Fuel
Impulse
Intake Air
Engine Vacuum
Start
Fuel
Impulse
Intake Air
Engine Vacuum
Idle
Fuel
Impulse
Intake Air
Engine Vacuum
Intermediate
3-1
3-2
3-3