I just got a 1985 300zx about 4 days ago, it ran pretty well on the hour drive home and the 30 minute trip back and forth to work that day. It started running rough when we took it for another ride the same night, and it sounded like the timing belt was close to breaking.

Upon changing the timing belt the next day, we found that it was so loose that it was flopping around, making it sound like it was ripping apart. While we were changing the timing belt we also changed the cap + rotor, plug wires (plugs had been done recently), belts, thermostat, and the oil. It idled fine, but when we got it onto the road, it would take about 10 seconds to go from 25-35 mph (ran sluggish through all other gears + speeds, but thats where its the worst).

We dumped a bottle of Heet (like isodry) in a full tank of gas, figuring if the gas had water in it it wouldn't fire right.

If someone has any other suggestions on what to do, that would be great.

 

better hope your timming gears are still in correct order.

 

If nothing mechanical is wrong with it research "Sea Foam" and run that through your car, cleans all the carbon build ups out. I just picked up a bottle of my own today and i plan to run it through once i can figure out where my Brake booster lines are.

 

well, after a bit of experimenting and poking it a bit, i think i found the problem. While the car was running i unplugged the mass air flow sensor and nothing happened.... So im guessing its that.

 

If you unplug the Mass Airflow and rev up the engine, it should stall. That means it is working. On the other hand your timing may be off, you should check that.
E-mail me and I will give e-mail you on trouble shooting the mass airflow this evening.