There's an embarrassing surfeit of fuel depots along the Merzouga to Tagounite route. Typically you will pay 15 dh/litre instead of 9.3 but I'm happy to do that. Good quality fuel, often transferred using a fuel filter inline.

Jadid: N30 50.891 W4 09.766
Ramlia: N30 41.189 W4 24.936
Tafraoute Sidi Ali: N30 41.332 W4 40.991
Oumjrane: N30 38.474 W5 06.304
Tissamoumine: N30 36.202 W5 09.655

There's a Ziz north of Mhamid at N29 59.352 W5 35.054, then several fuel depots in Mhamid to top off if you want. No fuel stations between Mhamid and Tata (via piste) though there's a line of five auberges around N29 56.145 W6 24.390 that you could ask.

Also no fuel stations at Mrimina or Tissint though again if you ask around you will probably find some for sale—clearly the guys on their small capacity bikes won't be riding all the way to either Tata or FZ in order to stick a couple of litres in their tank.

Another alternative would be to fill a couple of 1.5-litre water bottles with fuel then dump the contents into the tank after 50 and 100 km (and then chuck the bottles but put the tops on before you do, someone will reuse them). After you get to the divergence of the FZ and Tata pistes at N29 43.546 W6 51.073 there is a chance you might get turned back by the military in which case head for FZ via N29 48.632 W6 56.223

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"For sheer delight there is nothing like altitude; it gives one the thrill of adventure
and enlarges the world in which you live," Irving Mather (1892-1966)