Cummins/Onan 12.5JC Generator Set

⚠ Air housing door must be on when operating engine

Tools: Screwdriver

1. Loosen power element mounting screws in slotted holes
2. Slide assembly until plunger rests on roll pin with shutter closed
3. Shutter must be closed at ambient temperature
4. Test element by heating: plunger moves out at about 120F
5. Total plunger movement should be at least 13/64 in

⚠ Do not force needle valves against seats

Tools: Screwdriver

1. Allow engine to warm up
2. At no load turn idle adjustment out until speed drops slightly
3. Turn needle in until speed returns to normal
4. Apply full load, turn main adjustment in until speed drops
5. Turn needle out until speed returns to normal

⚠ Oversensitive adjustment may cause hunting

Tools: Reed frequency meter, Wrench

1. Hold governor spring stud, turn nut to set speed
2. Use a reed type frequency meter on generator AC output
3. Prior Spec R: turn spring stud counterclockwise for more sensitivity
4. Adjust for maximum sensitivity without hunting
5. Begin Spec R: turn speed nut clockwise to increase speed
6. Begin Spec R: turn ratchet nut counterclockwise to increase spring tension

Tools: Feeler gauge

1. Adjust valves in firing order 1-2-4-3
2. Set timing and clearance for cylinder 1
3. Turn flywheel 180 degrees for cyl 2, between 10 and 45 deg past BC
4. Adjust cylinder 2 clearance
5. Turn 180 degrees for cyl 4, between 10 and 45 deg past TC, adjust
6. Turn 180 degrees for cyl 3, between 10 and 45 deg past BC, adjust

⚠ Set clearances at room temperature, at least two hours after shutdown

Tools: Feeler gauge, Torque wrench

1. Turn flywheel to compression stroke after intake valve closes
2. Align TC mark with timing pointer, then turn clockwise 10 to 45 degrees
3. Check cylinder head bolt torque
4. Check rocker-arm-to-valve clearance with feeler gauge
5. Adjust with locknut to Table 3 clearance

⚠ Early model four-cylinder engines do not have a BC mark on the flywheel

Tools: feeler gauge

1. Adjust valve clearance in firing order (1-2-4-3) sequence
2. Set timing for number 1 cylinder and adjust the valve clearance
3. Turn flywheel clockwise 180 degrees for number 2 cylinder (between 10 and 45 degrees past BC mark)
4. Adjust valve clearance for number 2 cylinder
5. Turn flywheel clockwise 180 degrees for number 4 cylinder (between 10 and 45 degrees past TC mark)
6. Adjust valve clearance for number 4 cylinder
7. Turn flywheel clockwise 180 degrees for number 3 cylinder (between 10 and 45 degrees past BC mark)
8. Adjust valve clearance for number 3 cylinder

Tools: feeler gauge, torque wrench

1. Turn flywheel until cylinder to be adjusted is on its compression stroke
2. Turn flywheel until TC mark lines up with timing pointer, then turn clockwise an additional 10 to 45 degrees
3. Check cylinder head-bolt torque (see Fig. 60)
4. Using a feeler gauge, check clearance between rocker arm and valve
5. Increase or reduce clearance until proper gap is established, adjusting with the locknut
6. Refer to Table 3 for correct valve clearance setting

⚠ Do not force needle valves against their seats; this will bend the needle.

Tools: Screwdriver (adjusting needles)

1. Be sure ignition and governor are adjusted and allow engine to warm up.
2. With no generator load, turn idle adjustment out until speed drops slightly below normal, then in until speed returns to normal.
3. Apply full load and carefully turn main adjustment in until speed drops slightly below normal, then out until speed returns to normal.

⚠ An over-sensitive adjustment may result in a hunting condition.

Tools: Wrench, Reed type frequency meter or mechanical tachometer

1. Adjust engine speed by turning governor speed adjusting nut; clockwise increases speed, counterclockwise decreases.
2. Adjust sensitivity by turning the sensitivity adjusting ratchet nut through hole in the blower housing.
3. If speed drops too much at full load, turn ratchet nut counterclockwise to increase spring tension.
4. After adjusting, replace dot button in blower housing and secure speed stud lock nut.

⚠ Adjust breaker point gap before timing the ignition.

Tools: Continuity test lamp, Feeler gauge

1. Remove air housing access door and breaker points cover; disconnect ignition lead and install continuity test lamp and battery.
2. Rotate flywheel clockwise until TC mark approaches timing indicator, then slowly until light goes out (points open).
3. If incorrect, align 50 ATC mark with pointer, loosen breaker plate capscrews and rotate plate until light goes out.
4. Tighten plate and recheck timing, then connect ignition lead and install cover.

⚠ Advance 10 degrees more for gas fuel.

Tools: Test lamp, Battery, Feeler gauge

1. Disconnect low voltage distributor lead and connect test lamp and battery so lamp lights with points closed.
2. Remove number 1 spark plug and rotate flywheel clockwise until air is forced from the plug hole.
3. Continue rotating slowly until lamp goes out; check TC mark alignment with timing pointer.
4. To adjust, align TC mark with pointer, loosen distributor body and rotate clockwise if early, counterclockwise if late, until light goes out; tighten.

⚠ With access door open engine overheats rapidly; run a few minutes without load only

Tools: Automotive timing light

1. Install automotive timing light on a spark plug lead
2. Run engine at rated speed and check timing
3. Loosen breaker plate mounting screws to correct timing
4. Rotate clockwise to advance, counterclockwise to retard
5. Tighten plate and recheck timing point