Bumps on outer walls consistantly affecting lower layers

Question

I recently purchased a Creality Ender 3 Max as my first 3D printer and overall I am quite impressed with the print quality after only leveling the bed.

However, every print I have done so far has had bumps on the outer walls, limited to a section of the lower layers, usually around 6 layers in height.

Standard print settings:

• 1.75 mm PLA filament
• 0.2 mm layer height
• 20 % infill

I have tried:

• Prusa Slic3r (default settings) vs Cura for my slicing. Prusa Slic3r exaggerated the effect compared to Cura.
• Increasing the extruder and heated bed temperatures (200 °C & 50 °C, 210 °C & 60 °C, 225 °C & 70 °C). Higher temperatures seem to reduce the effect.
• Reducing the print speed from 50.0 mm/s to 35.0 mm/s. Slower print speed seems to reduce the effect.

My next experiment will be to combine a higher extrusion temperature with reduced print speeds however I suspect that this is just making the print more forgiving and masking a different cause.

Does anybody know of a way I can eliminate these artifacts?
Further experiments I can run to debug this or even the correct terminology or search terms for this (I'm having difficulty finding anything that looks like this problem online) would also be greatly appreciated.

Updates:

• Test print with 0.1 mm Z Offset shows a slight improvement but does not resolve the issue.
• Test print with 0.2 mm Z Offset shows no improvement over 0.1 mm Z Offset and nearly compromises bed adhesion.
• I printed a 20 mm cube benchmark. X and Y measurements are spot on but the Z axis measures 19.2 mm.

Answer 1

I think I found an answer.

When the gantry is low (Z less than 100 mm high) the Xmax (right hand) side of the gantry has some play, if I move the gantry higher the rollers tighten up.

I went through Luke Hatfield's X Gantry Rework (PDF section 4.1.2, video tutorial), making deviations for my own machine (the vertical extrusions are mounted to the side of the base on the Ender 3 Max rather than the top).
Tightening the rollers using the eccentric bolts and trying to get the gantry as level as possible seems to have improved the issue immensely.

The Z measurement for the 20 mm cube benchmark is now 19.7 mm (0.5 mm improvement)

And the bumps on the boats are a lot less noticeable.

While reassembling I had some difficulty lining up the screws on the top horizontal extrusion, which would suggest that the vertical extrusions are not aligned correctly as suggested in item 1 of this post discussing problems found on a stock Ender 3 Max:

The vertical extrusions are out, there appears to be a fault in the design and the extrusions are about 2.2mm closer together at the bottom than at the top.. what this means is if you move the carriage to half way up and adjust the wheels perfectly, when you move it to the top the outside wheels will be too tight, and when you move it to the bottom the inside wheels will be too tight!

https://www.reddit.com/r/Ender3Max/comments/qa93m6/problems_i_have_found_with_stock_ender_3_max_and/
The suggestion in that post is to install a printed 2.2 mm shim between the vertical extrusion and the base. I'll give the shim fix a try at a later time, for now the quality of the prints is good enough.