John Deere stx38 - Manual (Page 52)

3 - 20
4/23/97
TESTS AND ADJUSTMENTS ENGINE
SLOW IDLE SPEED ADJUSTMENT
NON CARB/EPA (EARLY MODELS)
Reason:
To set engine slow idle mixture and rpm.
Equipment:
• JTO5719 Photo Tachometer
Procedure:
1. Move transaxle to neutral. Engage park brake.
Standard Carburetors—Non CARB/EPA
2. Put reflective tape on outer rim of blower housing
screen.
IMPORTANT: DO NOT force the SLOW idle mixture
screw tight, this will damage the needle and
seat. Air cleaner assembly removed for photo
clarity, DO NOT remove for tests.
3. While engine is OFF, turn SLOW idle mixture screw
clockwise (in) until lightly seated, then turn
counterclockwise (back-out) one full turn.
4. Start and run engine at MEDIUM idle for 5 minutes
to obtain operating temperature.
5. Move throttle lever to SLOW idle position.
6. Use a photo tachometer to check engine rpm at the
blower housing screen reflective timing tape.
7. Turn SLOW idle stop screw in either direction until
specified SLOW idle speed of 1650 ± 75 rpm is
reached.
8. Turn slow idle mixture screw counterclockwise
(back-out) until engine speed drops off (rich
mixture)—Note position of screw at rpm drop-off
point.
9. Turn slow idle mixture screw clockwise (in) until
engine speed increases and then drops off
(lean mixture)—Note position of screw at rpm
drop-off point.
10. Turn slow idle mixture screw counterclockwise
(back-out) until midpoint (1/8 to 1/4 turn)
between rich and lean rpm drop-off points is
reached.
NOTE: It may be necessary to adjust slow idle stop
screw again.
11. Recheck slow idle stop screw setting.
Specifications:
SLOW idle stop screw setting. . . . . . 1650 ± 75 rpm
NOTE: For high altitude operation (above 4000 feet),
use high altitude carburetor kit, service part
number AM119687 (1994 and above) to
prevent over rich fuel mixture and black
exhaust smoke.
M55907
Slow Idle Stop Screw
Slow Idle Mixture Screw