[Cura Settings for Volcano with 0.8 mm nozzle]

I was having this same issue with the 1.2mm volcano nozzle.

 

By configuring the line width to be a bit larger than the nozzle it helps to minimize these gaps.

(im using 1.4mm line width)

 

also, there is a setting in slic3r under retraction settings called "extra length on restart"

I've found that setting retraction length to 0.1 and extra length to 0.5 helps.

 

i suspect that the volumetric pressure settings might help with this as well but i havent tried that yet.

 

i also recommend using at least the 1.3.0 version of slic3r.