Generac Power Systems NP-40G - Manual (Page 70)

Section 4.1- INTRODUCTION TO FUEL SYSTEM
Gaseous Carburetion
Gas at positive pressure is deiivered from the
fuei iockoff soiehoid to the iniet of the reguiator
(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 propor-
tionai to the amount of air that is fiowing.
The negative pressure at the carburetor venturi
acts on the reguiator diaphragm to puii the dia­
phragm toward the source of iow pressure. A lever,
attached to the diaphragm, opens a metering vaive
which aiiows gas to enter and flow through the
carburetor.
The greater the air flow through the carburetor
venturi, the iower 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 vaive opens.
The foiiowing requirements of the secondary
reguiator must be emphasized:
□ It must be sensitive to pressure changes In the
carburetor venturi throughout the entire oper­
ating range.
n it must positiveiy stop the fiow 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 permit gas fiow.
Leakage Testing
Whenever any lines, fittings or other compo­
nents 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 de­
termine if any leakage exists. No leakage is permit­
ted.
DANGER!
DO NOT USE FLAME TO CHECK FOR LEAKAGE.
GASEOUS FUEL LINES BETWEEN THE TANK
AND SECONDARY REGULATOR ARE UNDER A
POSITIVE PRESSURE (ABOUT 11 INCHES OF
WATER COLUMN). HOWEVER, GAS PRESSURE
AT THE OUTLET SIDE OF THE SECONDARY
REGULATOR IS A NEGATIVE PRESSURE
(ABOUT 1 INCH WATER COLUMN). THIS NEGA­
TIVE PRESSURE CAN DRAW FLAME INSIDE A
LINE OR FITTING AND CAUSE AN EXPLOSION.
IMPORTANT!
APPLY PIPE SEALANT TO THREADS OF ALL
FITTINGS TO REDUCE THE POSSIBILITY OF
LEAKAGE.
Page 4.1-3