The carb diaphargm contains the fuel pump to pump fuel into the small well into the fuel tank, as well as the vacuum chamber to operate the auto choke. So you replace all the parts with just the diaphragm. the choke may be opening and closing instead of staying open when it is running. Or maybe not pumping fuel correctly, and causing it to run lean. Clean the carb while you have the tank/carb off the engine.
Just make sure to read the instructions that comes with the new diaphragm to preset the choke correctly.
