Onan manufactures a line of small engines that are commonly used on small generators. These Onan-powered generators are often found on recreational vehicles or in other instances where consistent electricity is needed to operate a few electric lights or appliances. An Onan engine is protected by a governor that will increase the speed when a greater load is applied, or reduce the RPMs when under a lesser load. It takes only basic hand tools to adjust the governor on an Onan engine to keep it running at the optimal level.
Turn off the Onan engine. Inspect the linkage between the carburetor's throttle control and the governor arm. The linkage should move smoothly and the springs should not be overly stretched. If either appears damaged, replace with genuine Onan parts
Start the engine, and allow it to warm up for about 5 minutes.
Place the engine under a light-to-moderate workload.
Locate the spring on the side of the governor arm, about two-thirds of the way down from the end of the arm. Follow this spring back to a bolt through a mounting flange, with a nut on the opposite side. This is the speed adjustment nut. Turn this nut with an open end wrench and pay attention to the engine speed. As you tighten the nut, the engine speed should increase, and conversely, as you loosen the nut, the speed should decrease. Adjust this nut until your engine is at the desired speed.
Reduce the load, and allow the engine to idle for about 30 seconds, then put the engine under a full load.
Locate the screw in the governor arm. Turn the screw until the engine is running as smoothly as possible at full load, then allow the engine to idle. The engine should idle down smoothly with no sputtering at any point. Make adjustments to this screw until the engine runs smoothly at full speed and at idle.