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

CHAPTER 3 CARBURETORS AND FUEL SYSTEMS

GENERAL INFORMATION

TC/TM engines almost exclusively use diaphragm-type carburetors to be able to run effectively at any operating angle.
The diaphragm carburetors are produced by Tecumseh, 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 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, Microfiche Catalog, or
Electronic Parts Catalog, a cross reference chart will
convert a carburetor manufacturing number to a
Tecumseh part number. When using computer parts look
up system, it is only necessary to enter this information in
the search box using prefix OK for Tecumseh carburetors,
WA or WTA for Walbro carburetors or HU for Tillotson
carburetors (diag. 3-1).

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, upward piston
travel creates a low pressure area in the crankcase. High
pressure atmospheric air rushes into the crankcase 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. 3-2).
Fuel
Impulse
Intake Air
Engine Vacuum
Start
3-1
3-2
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