The most common reason for this issue will be your carburetor. If it is dirty, it won’t let in enough air into the engine. Access the carburetor, remove it from your generator and disassemble it for a thorough cleaning. Be sure to clean out the main jet and all the ports using carburetor cleaner, compressed air, and tip cleaners. Clean up the bowl of any buildup as well. Before you reattach it to the engine, check the gasket to be sure it is in good shape and if you notice it is worn, then replace it. Reinstall the carburetor and start up the generator, switching the choke off slowly.
The carburetors are typically the most common problem piece in generators, so keeping them clean will help you avoid headaches.