Changed oil in 1983 nissan 280 zx . Now running rough and smokes a little has 280000 miles

if this is a carby engine then the hesitation can be from a faulty power valve in the carby (Carby overhaul) or incorrect timing . It could be an indication that the turbo is not spooling up quickly enough to produce the air required on acceleration Check for any split/loose vacuum hoses that are altering the mixture

CHECK YOUR FUEL PUMP

Just letting you know that white is water, blue is oil and black is gas coming out of the exhaust.

Sounds like you could have an aux air regulator valve, that is not working when it will not work; there is no fast idle cold. Should be around 1500 rpm cold, the car will stall out and pop through the intake when bad. You have a digital volt meter to do some testing with to make sure the mass air flow is working correctly.

Also, check the plugs for how they're burning, check the air filter for good flow, do a compression test and judge from there. Intake backfire is usually caused by running lean or improper timing.

For the 79 model zx, ignition timing was 10 degrees BTDC @ 700rpm
For 80 and up zx, non turbo - 8 degrees BTDC +- 2 degrees @ 700rpm
turbo - 20 degrees BTDC +- 3 degrees @ 700 rpm

Also done with distributor vacuum advance hose removed and plugged.
NOTE:
* BTDC = Before Top Dead Center
* + - = Plus or minus
Hope thos helps; keep us updated.

Sounds like poor contacts. Is the other side bright?

1979?' isnt this a carb model??

There are 4 bolts for each seat, I think the head of the bolts are 14mm. Two in the front running parallel to the floorpan and the two in the back are vertical. Remove those and the seat comes right out. Todd in Idaho

sounds like a computer problem to me.

CATALYTIC CONVERTER MAYBE! BE MY GUESS AND IT ONLY HELPS THE TRUCK TO HAVE IT CHANGED!