I replaced the fuel regulator. And the extraordinary heavy fuel consumption and clouds of black smoke went back to "normal".
By normal, i mean ~10 mpg and black smoke whenever the revs above 4000 rpm.
Another problem has arised (it'll never stop). When i touch the gas pedal from a full stop, the engine surges and half of the times completely dies. -Very annoying!
The only thing i can think of is the little microswitch that is released when activating the gas pedal. Could this switch cause a surge like this if it's bad?