I have been having a problem with stringing on my 3D printer, so initially I increased retraction distance from 6.5 mm to 8 mm. Then I tried again, with the same issue, so I increased retraction speed and lowered the extruder temperature. Same problem. Over and over. After I did each of these, I have since decreased move speed, increased minimum layer time, put fan on full blast, and nothing has seemed to increase print quality.
I realize this is a lot of alterations to be making, but I'm not sure what to do next. I am using 1.75 mm PLA and a Monoprice Select V2.
Stringing can be fought by increasing the retraction length, but 6.5 mm (the default in Ultimaker Cura) is already pretty much. Please do note that too much retraction can lead to sucking air into the nozzle, which can lead to air pockets/holes.
Basically there are 3 mechanisms that are in play:
You have tried changing multiple parameters already. You may have better luck in reducing the temperature of the nozzle even further, start with a decrease of 5 °C. All that is left to do is increasing the travel speed.
There are many stringing test prints to print to fine tune stringing.