How do I send MIDI properly with ESP8266?
NOTE : Probably not the answer you're looking for, but it works for me. 

Solution : Move the jumper from 5V to Vin. 
Why? I don't know. It just works. 

Please notice that this is Wemos D1 Uno, which has adaptor input, which can receive 9-24V. 
If you're going to use the adaptor, IT MAY BROKE YOUR MIDI DEVICE!
As far as I know, Vin stands for Voltage Input, and everything from the adaptor is routed there. That means if you use 24v adaptor, Vin is 24v!

I'm only planning to use USB as power supply, so yeah. it works for me. 
If you're going to use Vin with external power supply, please use voltage regulator to 5v. Better safe than sorry. 

Anyway, since I'm planning to use 6N137 to receive MIDI also on this board, here goes : 


  Input voltages greater than 7 V must be avoided to preclude damage to
  the gate oxide of the input stage. This damage is not necessarily
  permanent, but will adversely affect the expected lifetime of the
  circuit. The gate oxide of AHC devices is only 200 Å thick. An input
  voltage of 7 V corresponds to a field strength over the gate oxide of
  350 kV/cm. Although breakdown of the oxide is expected only at input
  voltages above 10 V, electrons tunnel increasingly into the gate oxide
  at field strengths greater than 350 kV/cm, influencing characteristics
  of the transistors and causing failure.


Source : this question on electronics stackexchange.