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3-8
TROUBLE DIAGNOSIS
ENGINE FUEL AND EXHAUST SYSTEMS
(82)
since the automatic choke has been designed
and calibrated to provide easy and positive
starting with fuels of ordinary
volatility,
but
if the owner wishes to use volatile gasolines the
automatic choke thermostat should be adjusted
for a "lean" setting (par. 3-13).
3-6 IMPROPER ENGINE PERFORMANCE
a. Engine Idles Too Fast
A cold engine should operate on fast idle for
two to five minutes depending on air tempera-
ture. At
32°
F. the fast idle cam should move
to slow idle position in approximately
1J2
to
%.
mile of driving. At higher temperatures it
should move to slow idle position in a corre-
spondingly shorter distance.
If
the engine operates too long on the fast
idle cam
,
check the choke thermostat setting
(par. 3-13) and the fast idle adjustment (par.
3-23, Carter, or par.
3-30,
Stromberg).
If
engine idles faster than 8 MPH on a level
road in third speed when off the fast idle cam,
check throttle linkage for binding or weak re-
turn spring and adj ust throttle stop screw
(par. 3-12).
b. Improper Idle and Low Speed
Performance
Rough idling and tendency to stall may be
caused by idling speed set below 8 MPH or im-
proper needle valve adjustment
(par.
3-12).
High fuel pump pressure will cause rough
idling and poor low speed performance (par.
3-17).
An intake manifold air leak will cause rough
idling and poor low speed performance. A mani-
fold air leak produces a low, erratic reading on
a vacuum gauge connected to the intake mani-
fold. Look for leaks at windshield wiper and all
pipe connections and check manifold joints with
gasoline.
Rough idling, poor performance, and back
firing at low speeds frequently originates in im-
proper ignition. Check ignition system (par.
10-44) .
When rough idling and poor low speed per-
formance cannot be corrected
by
checks of
car-
buretion and ignition
mentioned
above, check
valve adjustment and check cylinder com-
pression.
c. Improper High Speed Operation
Roughness or poor performance above 22
MPH indicates faulty ignition (par. 10-44) or
improper settings in the high speed circuit of
carburetor.
With
Carter carburetors,
a surging or loss of
power at 55 to 65 MPH constant speed indi-
cates that the metering rod adjustment is too
lean. This may occur even though top speed
performance is satisfactory.
W
ith Stromberg carbureto
rs,
surging at 75
to 80 MPH constant speed indicates that the
power jet is stopped up or the vacuum piston
is sticking.
If
there is lack of power at top speed, check
throttle linkage to insure full throttle valve
opening (par. 3-10).
3-7 EXCESSIVE FUEL CONSUMPTION
Complaints of excessive fuel consumption re-
quire a careful investigation of owner driving
habits and operating conditions as well as the
mechanical conditions of the engine and fuel
system; otherwise, much useless work may be
done in an attempt to increase fuel economy.
Driving habits which seriously affect fuel
economy are: high speed driving, frequent and
rapid acceleration, driving too long in first or
second speed when getting under way, exces-
sive idling while standing.
Operating conditions which adversely affect
fuel economy are: frequent starts and stops,
congested traffic, poor roads, hills and moun-
tains, high
winds,
low tire pressures.
High speed is the greatest contributor to low
gas mileage. Air resistance increases as the
square of the speed. For instance, a car going
sixty miles an hour must overcome air resist-
ance four times as great as when going thirty
miles an hour. At eighty miles an hour the re-
sistance is over seven times as great as when
going thirty miles an hour. Over seventy-five
per cent of the power required to drive a car
eighty miles an hour is used in overcoming
air resistance, while at thirty miles an hour
only thirty per cent of the power required is
used to overcome air resistance.
Gas mileage records made by car owners
never give a true picture of the efficiency of the
engine fuel system since they include the effects
of driving habits and operating conditions. Be-
cause of the wide variation in these conditions
it is impossible to give average mileage figures
for cars in general use; therefore, any investi-
gation of a mileage complaint must be based on
an accurate measurement of gasoline consump-
tion per mile under proper test conditions.
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