Engine Tune-Up
DETROIT DIESEL
Fig. 7 - Adjusting Buffer Screw
Fig. 6 - Adjusting Idle Speed
installation or removal of shims, redheck the governor gap. If readjustment of the governor gap is required, the position
of the injector racks must be rechecked.
NOTE: Governor stops are used to limit the compression of the governor
spring which determines the maximum speed of the engine.
Adjust Idle Speed
With the maximum no-load speed properly adjusted, adjust the idle speed as follows:
1. Place the stop lever in the RUN position and the speed control lever in the IDLE position.
2. With the engine running at normal operating temperature, back out the buffer screw to avoid contact with the
differential lever.
3. Loosen the lock nut and turn the idle speed adjusting screw (Fig. 6) until the engine is operating at approximately 15
rpm below the recommended idle speed.
The recommended idle speed is 550 rpm, but may vary with special engine applications.
4. Hold the idle speed adjusting screw and tighten the lock nut.
Adjust Buffer Screw
1. With the engine running at normal operating temperature, turn the buffer screw in (Fig. 7) so that it contacts the
differential lever as lightly as possible and still eliminates engine roll.
NOTE: Do not increase the engine idle speed more than 15 rpm with the buffer
screw.
2. Hold the buffer screw and tighten the lock nut.
Adjust Booster Spring
With the engine idle speed adjusted, adjust the booster spring as follows:
1.Move the speed control lever to the idle speed position.
2. Refer to Fig. 8 and loosen the booster spring retaining nut on the speed control lever. Loosen the lock nuts on the
eye bolt at the opposite end of the booster spring.
3. Move the spring retaining bolt in the slot of the speed control lever until the center of the bolt is on or slightly over
center (toward the idle speed position) of an imaginary line through the bolt, lever shaft and eye bolt. Hold the bolt and
tighten the lock nut.
4. Start the engine and move the speed control lever to the maximum speed position and release it. The lever should
return to the idle speed position. If it does not, reduce the tension on the booster spring. If it does, continue to increase
the spring tension until the point is reached where it will not return to Idle. Then reduce
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