Changing central meridian in QGIS causes problems?

Question

I want to change the central meridian of my world map (WGS84) in QGIS from 0 to -80. The problem is that this distorts the map every time I try it.

This is what I do:

• go to Settings/Custom CRS and then click on Add new CRS
• copy existing CRS (choosing WGS84 / World Mercator)
• Edit its parameters (changing only the central meridian) to: +proj=merc +lon_0=-80 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
• go to Project Properties/CRS and choose my custom CRS

And then the whole map is distorted. It works fine when I do it in ArcGIS.