GMC 1976 ZEO 6083 Maintenance Manual page 290

delivery system between the supply tank and the
carburetor . Refer to "Fuel Pump and/or Carburetor
Diagnosis" charts in this manual . Insufficient fuel
discharge, however, may also be due to the operation
of the accelerator pump circuit within the carbu-
retor.
9 . VACUUM LEAKAGE CHECK
With the engine at idle speed, squirt a mixture of
kerosene and 10W motor oil around areas where
vacuum leakage may occur. A noticeable change in
the engine idle when the mixture is squirted on a
given point indicates a vacuum leak at that point.
CAUTION :
Kerosene and oil mixture is
flammable. Careless application may result
in fire . DO NOT use gasoline.
10. EXCESSIVE FUEL
CONSUMPTION CHECK
There are a number of factors, other than engine
or carburetor problems, that will contribute to exces-
sive fuel consumption. One of the most important of
these is the driving habits of the operator .
When the operator habitually makes "jack-rab-
bit" starts and stops, "rides" the brake pedal, over-
loads the vehicle, drives at excessively high speeds
for prolonged periods, fails to hold a consistent
throttle position. (continuously accelerates, then
coasts) and/or operates the vehicle under short run
conditions (cold engine) the majority of the time, this
could be the problem.
Vehicle air resistance at high speeds has a major
affect on fuel consumption. Head winds, excessively
high speeds, or added protrusions to the vehicle pro-
file will cause an increase in fuel consumption.
When it has been determined that the operator is
not at fault, make a fuel consumption test using a
calibrated fuel measuring device . The amount of fuel
used to drive the vehicle a measured distance should
be recorded . Then record the amount of fuel used to
return to the starting point . An average of the two
readings should be used in determining the existence
of a fuel consumption problem. While making the
fuel consumption test, the vehicle odometer should
be checked over a measured mile for proper calibra-
tion .
ENGINE 6A- 25
If the results of the fuel consumption test indicate
that a fuel consumption problem does exist, the diag-
nostic procedures outlined in this manual under
"Excessive Fuel Consumption in Carburetor" and-
/or "Ignition Diagnosis" charts should be followed .
11 . VAPOR LOCK CHECK
The term "vapor lock" means the flow of fuel to
the mixing chamber in the carburetor has been
stopped (locked) by the formation of vaporized fuel
pockets or bubbles caused by overheating the fuel by
hot fuel pump, hot fuel lines or hot carburetor .
The more volatile the fuel the greater the tend-
ency for it to vapor lock . Vapor lock is encouraged
by high atmospheric temperature, hard driving, de-
fective engine cooling and high altitude .
A mild case of vapor lock will cause missing and
hard starting when engine is warm ; also a "sag"
during an acceleration or surge during cruise . Some
what more severe vapor lock will stop the engine
which cannot be started again until it has cooled off
enough so that any vaporized fuel has condensed to
a liquid .
IMPORTANT :
Percolation means simply that
gasoline in the carburetor bowl is boiling over
into the intake manifold . This condition is most
apt to occur immediately after a hot engine is
shut off. The carburetor has provision for reliev-
ing the vapor pressure of overheated fuel in the
carburetor bowl by means of internal vents. If,
however, percolation should take place, the en-
gine may be started by allowing it to cool slightly
and then holding the throttle wide open while
cranking to clear the intake manifold of excess
fuel .
Some causes of vapor lock may be due to winter
grade fuel used in summer (most vapor lock occurs
in April due to this), or temperature under hood is
too high .
NOTE:
Applying wet cloths to fuel lines, fuel
pump and/or carburetor can cause fuel to con-
dense and permit engine to run.