The air/fuel screw usually has a little o-ring sealing it. If this o-ring fell out or is damaged, air will get sucked in. When you check for leaks, spray carburetor cleaner around the couplings and joints in addition to that screw while the engine is idling. If the idle changes, you found a leak.
Since your carburetor has a manual choke, make sure that the mechanism is working properly. It will not hurt for you to check for leaks here also.
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