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.
The STIHL 084 AV is a professional-grade, high-capacity chainsaw manufactured between 1985 and 1997 stihl 084 av service manual
If you can find a free PDF scan online, print it yourself. Don’t pay a premium for a bound copy unless you are a collector. It gets the job done, but it is not a joy to use. Turn to the "Fuel System" section