If you want to keep your 084 AV cutting firewood for another 30 years, buy this manual. It paid for itself the first time I didn't have to pay a shop $150 to diagnose a "no spark" issue (turned out to be a dirty ground wire—found in Section 5).
Turn to the "Fuel System" section. The manual specifies that the fuel line is part number 1119 358 7700 and has an inner diameter of 3.0 mm. You measure yours; it's swollen shut. Replace it.
While Stihl does not officially distribute free digital service manuals to the public, original and reproduction physical copies are widely available through secondary markets [7, 8].