It sounds like lean mixture then, and you will need to look at the carburetor. Please post pictures of the carburetor. It appears to be a version of the Pulsa-Jet with governed idle and a rotary choke, but let's be sure we are on the same page about that.

Once we are both sure which carburetor we are talking about, you will need to remove the carburetor and attached fuel tank from the engine, then remove the carburetor from the fuel tank. Turn the carburetor upside down and check that the fuel screen is clear (circled in red in this illustration from the manual):

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If the screen is clear, we can look at the main jet, but the screen is the usual cause of your kind of problem. If there is dirt or, more likely, grass on the screen, you will need to remove the fouling. I do not recommend following the instruction on the diagram to routinely cut the nylon tube, since you'd then need a new screen: usually you can avoid this disassembly process and clean the screen from the outside with a soft brush, being careful not to push the rubbish through the screen.