Cummins/Onan 5.5 NH / 6.5 NH — Mixture Screw Adjustments (Gasoline Carburetor)
Adjustments
For the Cummins/Onan 5.5 NH / 6.5 NH, from the Cummins/Onan operator's manual.
Applies to: Cummins/Onan NH engine
Before you start
- Forcing the mixture adjustment screws tight will damage the needle and seat; turn in only until light tension can be felt.
- T-handled main adjustment screws are locked by a packing nut; loosen before adjusting and retighten afterward. Failure to tighten the packing nut might result in leaking fuel, creating a serious fuel hazard.
Procedure
- Remove all electrical loads and connect a voltmeter and frequency meter to the generator set.
- Pull the governor linkage toward the front of the set so the throttle lever rests against the throttle stop screw; adjust the stop screw for 90 to 100 volts.
- While holding the governor linkage, turn the idle adjustment screw inward until voltage drops, then outward until it drops again; set slightly rich from the midpoint.
- Release the governor and apply full load; set the main adjustment screw the same way, slightly rich from the midpoint.
- Remove the load and check stability; adjust governor sensitivity; add and remove full load several times to be sure the set does not bog down or hunt.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Preliminary mixture settings, Standard and Contractor models: Idle 1-1/4, Main 1-1/4
- Preliminary mixture settings, Mobile model: Idle 7/8 - 1-1/8, Main 1-1/4 - 1-1/2
- Throttle stop screw setting 90 to 100 volts
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.