I have a 1976 280Z, it starts up cold just fine, runs great. But, after I turn it off for about 15 to 20 mins, it will run really rough and then smoth out after about 45 sec to a minute.

Any advise?

Thats an interesting problem, any more symptoms you can mention for us? FYI theres another club member who live in your town too if you didn't know.

Edited February 27, 201016 yr by WingZr0

Fuel is getting into the cylinders.Probable cause(s) in order:

Leaking injectors

Leaking Cold start valve

Thermotime switch

Could be a little heat soak going on, nearly vapor locking , and after the pump brings fresh cooler fuel up, it goes away. Just a thought. Also, make sure you are not losing coolant. If you are leaking into a cylinder, it will run rough until you purge all the water out.