Carburetors And Fuel Systems; General Information; Float Style Carburetors; Operational Circuits Series 7 Carburetor - Tecumseh V-twin Technician's Handbook

CHAPTER 3. CARBURETORS AND FUEL SYSTEMS

GENERAL INFORMATION

The TVT engine uses two series seven (7) float type
carburetors. This carburetor uses a choke enrichment
system to provide easy cold engine starting. To comply
with emission standards, the carburetor idle and high-
speed fuel mixtures are non-adjustable. Carburetor
cleaning and related fuel system service is covered in
this chapter.

FLOAT STYLE CARBURETORS

A float is used to maintain the operating level of fuel in
the carburetor bowl. As the engine consumes fuel, the
fuel level in the carburetor bowl drops and the float moves
downward. This allows the inlet needle valve to move off
the sealing seat, and fuel to enter the carburetor float
bowl. As the fuel level in the bowl rises, it elevates 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 keeping the supply
constant. (Illust. 3-1) The float height on the series 7
carburetor is fixed and may not be adjusted.
NOTE: Gravity fed systems must have the bottom of
the fuel tank no lower than the fuel inlet of the carburetor.
When servicing carburetors, use the engine model and
specification number to obtain the correct carburetor part
number or parts. An alternate method is to use the
manufacturing number stamped on the carburetor.
Convert this number to a service part number in Div. 5
carburetor section of the Master Parts Manual. This
method can also be used in microfiche and computer
parts look-up systems.
OPERATIONAL CIRCUITS SERIES 7
CARBURETOR
SERIES 7
IDLE
PRIMARY
IDLE MIXING
WELL
IDLE
RESTRICTOR
IDLE/TRANSFER
FUEL
PASSAGE
Choke Circuit: In the "CHOKE"/"START" position, the
choke shutter is closed, and the only air entering the
INLET
engine, enters through openings around the shutter. As
NEEDLE
the starting device cranks the engine over, the pistons
travel downward on the intake stroke, creating a low-
pressure area in the cylinder. High-pressure
(atmospheric) air rushes into the cylinder to fill the low-
pressure area created.
Since the choke shutter blocks the majority of the air
passage, a relatively small quantity of air enters the
carburetor. The main nozzle and idle fuel discharge ports
are all supplying fuel at this point. This is due to the
3-1
increased low air pressure in the intake system of the
engine. A maximum fuel flow through the carburetor
orifices combined with the reduced quantity of air that
passes through the carburetor, creates a very rich fuel
mixture needed to start a cold engine.
IDLE PROGRESSION HOLES
IDLE
AIR BLEED
VENT PASSAGES
MAIN JET
SPRING
ATMOSPHERIC
MAIN NOZZLE
AIR BLEED
3-2
3-1