Just bought an 84 Datsun 300zx, the only issue is it wont go over 30mph. I dont believe the transmission needs to be replaced but I'm at a lost.

Welcome to NICO. I moved your thread to the correct area.

A speed related issue is an unusual one. First fo all auto, or manual? does gear selection allow you to run higher rpm but still limiting the car speed?

I just bought an 86 300ZX 2+2 NA, auto tranny and it wont go into 3rd gear most of the time. Top speed it'll do is about 28-30mph. The tranny fluid is BLACK but it dont smell burnt as far as i can tell. I'm going to change the fluid and clean out the tranny filter by spraying with brake fluid. I'll probably repeat this about 3 times after every 100 miles then it should be clean. While I'm under the car draining the fluid I'll check the tranny vacuum module. If tranny fluid is in the hose or dripping from the module then the module is bad. Some Z's use the vacuum module to help determine when to shift. Lastly, If that don't do it then I'm going to check the kickdown cable to see if it needs adjustment. I've been researching for a few days and that's all the info on possible causes it may be.....without getting it rebuilt. Good luck, I hope one of these fixes it for you.