The passage runs from the very front of the carb (where the air flows inward) at one of two small openings facing the air filter. (There's a metering orifice there that really shouldn't be messed with. I think it's a press-fit, but there's no purpose in removing it anyway.)
The passageway runs from there to the opening (pickup) for the pilot, and then to a small pinhole-sized opening in the back of the carb -- so when you apply compressed air anywhere, there are two escape paths for it. This means it's ineffective unless you block one opening and spray into another, forcing the compressed air to travel through the third and only remaining orifice.
If all you do is apply compressed air to the front opening, or at the pilot jet threads, it's just like plunging a double sink -- you're just moving air back and forth through the same unclogged passage.
First, blow some compressed air into the pilot jet bore. Feel which opening at the front of the carb the air rushes out from. Hold a finger over that opening and spray again. You're NOW forcing (or trying to force) compressed air through the critical passageway.
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