Figure 4-53. Intake Manifold Vacuum Test

Figure 4-53. Intake Manifold Vacuum Test

(2150 RPM) which should be 14-16 inches.

one item may not increase the vacuum enough to

indicate that the trouble has been corrected. It is

f. With operator on seat and engine at

i m p o r t a n t , therefore, t h a t e a c h c a u s e o f a n

full throttle, hold tilt lever all the

abnormal reading be investigated and further tests

way back, and read vacuum which should

conducted where necessary in order to arrive at the

be 4 to 6 inches.

corrected diagnosis of the trouble.

g. Set parking brake firmly. Place transmission m

FORWARD range and accelerate engine to full stall

j . Allowance should be made for the effect of

s p e e d . At stall, vacuum should read 0 to l-1/2

altitude on the gauge reading. The engine vacuum

inches.

will decrease with an increase in altitude.

h . Manifold vacuum is affected by carburetor

4-52. FUEL MIXTURE AND IDLE SPEED

a d j u s t m e n t , valve timing, the condition of the

ADJUSTMENT.

valves, cylinder compression, and leakage at the

intake manifold, carburetor, or cylinder head

4 - 5 3 . INITIAL IDLE MIXTURE ADJUSTMENT

gasket.

(See figure 4-54). Set the preliminary idle mixture

b y t u r n i n g t h e i d l e m i x t u r e s c r e w in ward

i. Because abnormal gauge readings may indicate

(clockwise) until it is lightly seated, then turn the

that more than one of the above factors is at fault,

screw outward (counterclockwise) 1 to l-1/2 turns.

use caution in analyzing an abnormal reading. For

Do not turn the screw needle tightly against its seat

example, if the vacuum is low, the correction of

as this may groove the end. If the needle is

4-39