Diagnosis
Fuel starvation. The carburetor is getting inconsistent fuel flow, usually due to old gas leaving varnish or a clogged fuel filter.
How to Fix It
Drain all fuel from the tank and carburetor bowl. Replace the inline fuel filter. Add fresh, non-ethanol gasoline. Run the generator under a 1000W load for at least 30 minutes. The load helps clean the system.
If That Didn't Work
The carburetor jet itself is clogged. The carburetor needs to be removed, fully disassembled, and cleaned with carb cleaner and compressed air. The main jet is tiny and clogs easily.
Heads Up
Never store this generator with gasoline in the carburetor. Use the fuel shutoff valve or run it dry. Ethanol-blended fuel left to sit is the primary cause of this issue.