Cummins/Onan — Injection Pump Timing by Calculation
Adjustments
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Do not change the pump mounting shim total thickness or the proper pump gear to camshaft gear mesh will be affected.
Procedure
- Remove old pump.
- Determine port closing dimensions and original button thickness from old pump.
- Write down port closing dimension on old pump flange and on new pump flange.
- Remove tappet, retaining ring and timing button from old pump; obtain old button dimension from Table 1.
- Add dimension on old pump flange to timing button dimension.
- Subtract port closing dimension on new pump flange from total dimension for old pump.
- Select new timing button nearest the calculated dimension, install it and install tappet.
- Install injection pump.
Tools required
- Channel lock pliers or screwdriver Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Steel shims between pump and cylinder block maintain proper gear backlash
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.