One possibility...and this is a bit of a longshot...the carb body or/and slide are worn to a point where you're getting "false air". Only way to definitively rule this out would be testing with another, known, carburetor, probably not he easiest thing to do with this model. And a new carb is kinda pricey just to use as a testing device...especially when it may not be the problem. As an interim test, remove the air filter, then rev the engine, close the throttle, then cover the carb inlet with you hand. The revs should plummet, almost instantaneously. If the revs still trail-off gradually, then there's an air/vacuum leak somewhere in the system.
Another possibility is air leakage at the carb cap, above the slide. That's easy enough to check. Temporarily seal the the cap to the carb body using electrical tape; the same can be done to seal the throttle cable from the brass ferrule right up to the outer cable housing.