86 300zx not starting
 

I have a 1986 300 ZX non-turbo that will not start. I have had it since July 2017. It has been running fine until 2 weekends ago. Upon starting it was running really rough with black smoke coming out of the exhaust. I am assumed it was running rich. After I turned it off it would not start. I waited several hours tried again and it started, still running rough. On the ride home it suddenly started running normal. Drove it last week, went to work Friday and it hasn't started since. I ran diagnosis on ECU. It gave me codes 12, 13, 21, and 41. I replaced the Air Flow Sensor last night. It first tried to start, then nothing. It is now showing 23, 31, and 41. It has plenty of fuel. You can smell it. It has spark at the distributor. Thanks for your input.

Check the battery and connectors....these cars do weird things when amperage is low or going out.

Start simple.....then use deductive logic and FSM.