I guess its safe to say you plugged up that vent with JB Weld? On some of the 8000 tanks there is an inside filter built into the tank where the vent line attaches, from your pics this looks to be what your working with. On other 8000 tanks the vent line plugs into the tank via a separate vertical hole (like a cs-670) near the fuel line port. The purpose of the inside filter is to keep the liquid fuel out of the vent allowing the tank to take a breath when its moved from a vertical to a horizontal like when filing the tank or felling a tree. In addition to this vent there is usually a second vent located under the handle bar support located on the right side of the tank. To access the second vent remove the 3 bolts at the bottom of the right handle support, and one more at the carb box. Then the handle support can be swung to the side exposing the second vent.
What model no. is on the plate up under the clutch cover for this unit?
The qv/cs-8000 saw has been in circulation for 32 years. I think this is why Echo came out with the 800p so they could standardize all the parts. There are several different carbs, ignitions, pistons, fuel tanks, and at least 3 different vents used on the design.
Pull the recoil (4bolts) and the air vane underneath (lifts off), remove the top cover (4bolts). Move over to where your leak was originally and find the vent line. Pull the cap off the white jelly bean with a small flatblade and take out the filter (black square foam) and the duckbill (black rubber) with the tip of the blade. You can loosen or remove the air box to tank cushion on the recoil side by removing 3 more bolts that hold it on. Pay attention to the length of those bolts when you put it back together.
Once you get the line accessible, push some air through it with your mouth and see if it vents (make sure the fuel is out and the cap is off). If it vents, leave out the filter and duckbill, put the white cap back onto the jellybean and try a few cuts. The correct service would be to replace the carb diaphragms, both fuel lines, the top vent line with the jellybean, and the vent on the right side.