In my definition a proper JFC is supposed to be able to shut completely the ejuice from reaching your wick, as far as this condition is concerned Merlin Mini does not has a JFC.
Even though the 4 juice holes looked shut, juice still flow through as shown by red arrows in the diagram below:
If you unscrew a bit the tank from the base, you get something like below:
That makes it looked like 2 out of the 4 juice holes are fully opened. (In fact all 4 of the juice holes are intaking juice)
For MTL vaping, because juice comsumption rate is relatively lower compare to DTL, this step is rather unnessary.
For juice with high VG, DTL and chained vaping, then may be you should do the above.
To me "losen your tank from the base a bit" in order to have bigger juice flow is WEIRD.
If I did the WEIRD and leave my Merlin Mini (filled with juice) over 24 hours, I often found
seaping. (and it won't saep if I screw my tank very tight to the base, making all 4 juice holes looked shut).
This tank is a downgrade of the 4ml ordinary Augvape Merlin. They have sacrificed user-friendliness and practicality for the fancy short-and-fatty look.