If you had to keep it running by applying the choke, you likely have some dirt in a passage or the carb shook loose- especially if it developed as a problem quickly. The other less likely possibility is a crack in a reed valve.
The short answer is get a can of carb cleaner and spray around the mounting flanges or mating points of various induction parts as they attach to the engine. With the engine running. If you have a vacuum leak, you know you found it when the engine speeds up after an area such as the carb flange has been sprayed with carb cleaner. Just short shots of cleaner directed at the flanges and mounting points, and vacuum lines as well.
clean the carb second, when it is off the bike, look to see if there are any visible cracks on the metal things that seemingly block the manifold looking in to the manifold- the part the carb bolted to. These keep the crankcase presurized on the piston downstroke so the combustion chamber can get adequate fuel and air.
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