Underextrusion between walls

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Hello guys. I upgraded my Tevo Black Widow with a Titan Aero extruder and I am very pleased with it except for one issue I am currently dealing with.

 

For some reason I see seemingly random underextrusion, which however appears on exact same place when I print multiple times.

 

A short overview of my setup and settings:

Nozzle diameter: 0.4mm

Layer Height: 0.1

Wall Line Count: 4

Retraction: 1.1mm

Retraction Speed: 35mm/s

Skin Expand Distance: 0.8mm

 

Esteps calibration within 1mm on a 100mm extrusion test.

I tried to use some common sense from the options that Cura offers, so I increased flow rate to 101% to deal with the wall line separation and increased Skin Overlap Percantage from 5% to 10% to deal with the holes around the letters. The result was improving the letter but the rest of the print got noticable worse as it shows signs of overextrusion and were much rougher compared to before making the changes when the finish was super smooth.

 

Any ideas?