Cummins/Onan — Flow Timing the PSU Pump
Adjustments · fuel injection pump
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Procedure
- Install No. 12 timing button in the PSU pump.
- Remove delivery valve cap and holder; take out spring and replace holder and cap.
- Remove door panel, air cleaner and top sheet metal cover for access to flywheel marks.
- Remove No. 1 injector line; re-install with top end in pump outlet so the other end directs fuel into an open container.
- Disconnect governor linkage at ball joint and wedge control arm at maximum fuel position.
- Rotate flywheel counterclockwise to about 15 degrees before the timing pointer (compression stroke No. 1).
- Manually operate the fuel transfer pump until air-free fuel flows steadily.
- Continue pump operation while an assistant rotates the flywheel slowly clockwise.
- Stop rotation at the exact point fuel stops flowing from No. 1 line (one drop in 2 to 5 seconds), which is port closing.
- If port closing does not align with PC mark, calculate the required button thickness change (0.1 inch flywheel = 0.003 inch button) and install a new button.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 0.1 inch (2.54 mm) flywheel graduation = 0.003 inch (0.076 mm) button thickness
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.