Cummins/Onan GTA38 / GTA50 — Gas & Ignition Timing Adjustment Procedure
Adjustments · Fuel and ignition systems
For the Cummins/Onan GTA38 / GTA50, from the Cummins/Onan owner's manual.
Applies to: Cummins/Onan GTA38 engine, Cummins/Onan GTA50 engine
Before you start
- Never adjust the power valve screw more than 1/4 turn (90 degrees) at a time.
- Do not exceed 20 inches of water to the on-engine gas flow regulator.
Procedure
- Install water manometers at on-engine gas regulator and carburetor.
- Set gas pressure to on-engine regulator 10-20 in H2O at full load.
- Set carburetor gas pressure 4-6 in H2O at full load.
- Set initial timing: GTA38 20-24 DBTDC, GTA50 19-21 DBTDC.
- Install pyrometer below turbo flange, 0-30 psi gauge on manifold, oxygen meter in turbo outlet.
- Connect timing light to #1 cylinder.
- At full load, adjust both carburetor power valves for lowest exhaust temperature.
- Retard/advance timing in 2 degree increments to find lowest EGT and boost without RPM loss.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Gas to regulator 10-20 in H2O
- Gas to carburetor 4-6 in H2O
- Timing GTA38 FridayParts $14.33 20-24 KuduParts $85.00 FridayParts $110.92 DBTDC
- Timing GTA50 FridayParts $316.79 19-21 SinoCMP $119.00 DBTDC
Source: Cummins/Onan owner'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.