Cummins/Onan — Engine Timing Adjustment
Adjustments · ignition timing
From a Cummins/Onan operator's manual covering: G-1710, G-495, G-743, G-855, GTA-1710, GTA-743, GTA-855. Check it applies to your exact model before use.
Applies to: Cummins/Onan G-855
Before you start
- Lower BTU fuels may require higher main line pressure; do not exceed 20 inches water to Thermac inlet.
Procedure
- Install pyrometer in 1/4 inch pipe tapped hole below turbo mounting flange.
- Install mercury manometer on intake manifold.
- Connect low and high tension timing light to No. 1 cylinder.
- Adjust carburetor power valve to obtain lowest possible exhaust temperature.
- Retard and advance timing by two degree increments and repeat to confirm lowest exhaust temperature and lowest manifold vacuum.
- Confirm readings several times.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Naturally aspirated initial timing 30 to 32 deg BTDC
- Turbocharged 743 and 855 initial timing 24 to 28 deg BTDC
- Turbocharged 1710 initial timing 28 to 32 deg BTDC
Source: Cummins/Onan operator's 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.