Craftsman 143.991200 Specifications download pdf (Page 11)

CHAPTER 3 CARBURETORS AND FUEL SYSTEMS

GENERAL INFORMATION

Tecumseh overhead valve engines use float type carburetors. Float type carburetors use a hollow float to

maintain the operating level of fuel in the carburetor.

The float type carburetor will have a fuel enrichment system of either a primer or a manual choke to provide

easy cold engine starting. The carburetor fuel mixtures are either fully adjustable, partially adjustable, or

nonadjustable. Carburetor adjustments, cleaning, and related fuel system service is covered in this chapter.

Some Tecumseh engines utilize Walbro carburetors. Basic operation is very similar to the Tecumseh float style

carburetor.

Carburetors used by Tecumseh can be identified by a manufacturing number stamping on the carburetor as

illustrated (diag. 1).

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 stamped on the carburetor and

convert this number to a service 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 Service part number.

Complete carburetor replacement may be

accomplished by a standard service carburetor. A

standard service carburetor is a basic carburetor that

may require the use of original carburetor parts or

additional new parts to adapt to the specification. An

instruction sheet is provided with the new service

carburetor or see SERVICE in this chapter.

OPERATION

In the CHOKE or START position, the choke

shutter is closed, and the only air entering the engine

enters through openings around the shutter. As the

starting device is operated to start the engine,

downward piston travel creates a low air 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. 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.

At engine IDLE speed, a relatively small amount of

fuel is required to operate the engine. The throttle is

almost completely closed. A fuel / air mixture is supplied through the primary idle-fuel discharge orifice during

idle.

During INTERMEDIATE engine operation, a second orifice is uncovered as the throttle shutter opens, and

more fuel mixture is allowed to atomize with the air flowing into the engine.

During HIGH SPEED engine operation, the throttle shutter is opened. Air flows through the carburetor at high

speed. The venturi, which decreases the size of the air passage through the carburetor, further accelerates

the air flow. This high speed movement of the air decreases the air pressure at the main nozzle opening. Fuel

is forced out the main nozzle opening due the difference in the atmospheric air pressure on the fuel in the

carburetor bowl and the reduced air pressure at the main nozzle opening.

ALTERNATE LOCATION

FOR MANUFACTURING

NUMBER

CARBURETOR

DATE CODE

MANUFACTURING

NUMBER

FLAT

DOWN

CHOKE PLATE

89 4F5

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Craftsman 143.991200 Specifications Page 11

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