Get some wd40 or or parts cleaner in a trigger spray (not very smart to use it out of a aresol can with an engine running) and spray down the area where the carb meets the intake manifold, the intake manifold, and where the intake manifold bolts to the engine, if there is a change in engine speed or it blows some smoke, you found your leak., i know you said you inspected them but all it takes is a small crack, enough for air to get in, and that will cause the engine to run lean and surge, sometimes surging can also be an issue with the governor, not always a fuel issue.