Generac Power Systems 941-2 Service Manual page 75

GASEOUS CARBURETION
Gas at positive pressure is delivered from the fuel
lockoff solenoid to the inlet of the regulator (about 11-
14 inches of water).
As the engine piston moves downward on its intake
stroke, air is drawn into the area above the piston
through the carburetor venturi. A negative pressure is
created at the venturi which is proportional to the
amount of air that is flowing.
The negative pressure at the carburetor venturi acts
on the regulator diaphragm to pull the diaphragm
toward the source of low pressure. A lever, attached
to the diaphragm, opens a metering valve which
allows gas to enter and flow through the carburetor.
The greater the air flow through the carburetor ven-
turi, the lower the pressure at the venturi throat. The
lower the pressure at the venturi throat, the greater
the movement of the diaphragm and the more the
metering valve opens.
Figure 2. Gas Carburetion Diagram
The following requirements of the secondary regulator
must be emphasized:
• It must be sensitive to pressure changes in the car-
buretor venturi throughout the entire operating
range.
• It must positively stop the flow of gas when the
engine is not running.
• The slightest air flow through the carburetor venturi
must move the regulator valve off its seat and per-
mit gas flow.
INTRODUCTION TO FUEL SYSTEM
LEAKAGE TESTING
Whenever any lines, fittings or other components of
the fuel system have been removed and replaced, the
system should be carefully checked for leaks before it
is placed into service.
To check for leakage, start the engine and let it run.
Then use a soap and water solution or an approved
commercial leak detector solution to determine if any
leakage exists. No leakage is permitted.
DANGER! DO NOT USE FLAME TO CHECK
!
FOR LEAKAGE. GASEOUS FUEL LINES
BETWEEN THE TANK AND SECONDARY
REGULATOR ARE UNDER A POSITIVE PRES-
SURE (ABOUT 11 INCHES OF WATER COL-
UMN). HOWEVER, GAS PRESSURE AT THE
OUTLET SIDE OF THE SECONDARY REGU-
LATOR IS A NEGATIVE PRESSURE (ABOUT 1
INCH WATER COLUMN). THIS NEGATIVE
PRESSURE CAN DRAW FLAME INSIDE A
LINE OR FITTING AND CAUSE AN EXPLO-
SION.
IMPORTANT! APPLY PIPE SEALANT TO
!
THREADS OF ALL FITTINGS TO REDUCE
THE POSSIBILITY OF LEAKAGE.
Section 4.1
Page 4.1-3