I have had this problem on two separate k10s, caused by two different things. Assuming you have done the useual "service part" checks first like leads, plugs, dizzy cap and rotor arm etc...
The first was caused by an excessivly weak mixture caused by the second chamber in the carb being seized up.
Turn the small lever connected the the large vaccum diaphram at the front of the carb and look down the second chanber (the one without the choke flap) to see that the butterfly flap at the bottom is moving freely. If this is fine, check the condition of the vaccum lines, if you spot any splits or cracks then it's a simple case of replaceing those pipes.
BUT REMEMBER; some of the vaccum lines have flow restictors in them which you'll need to push out with a screwdriver and insert into the new pipes.
The second was cause by a leaking carb-to-adaptor plate gasket (also called throttle chamber), and a leaking gasket between the underside of the adaptor plate and the top of the manifold itself. The upper gasket allowed coolant and air to be sucked into the manifold, and the lower gasket simply allowed air to be drawn in.
Since you have changed the carb i would recomend changing these gaskets these gaskets if you haven't already.
They cost around a fiver each and are still available from the main nissan dealerships.
Top gasket (Insulator Carbu); 16174-19B00
Bottom gasket (K GSKT - THROT CH); 16174-19B01
