There are several possible causes for the symptoms you describe. Taking a logical approach to isolatiing the problem and not skipping any steps is the key to finding the cause.
Start from scratch; just because you checked something previously, does not mean that you may not have missed something along the way; it happens to all of us.
Start with teh basics: engine mechanical condition. If you have a mechanical fault, no amount of adjusting or tuning will help the engine to run any better.
(1) Compression test the engine. All cylinders should be above about 120psi, and all should test withing about 10% of each other. If one orXXXXXdifferent than the rest, or if all are low, then there is a mechanical problem such as a leaky valve, cam wear, or timing chain failure causing the rough running. If you suspect a leaking valve, a cylinder leakdown test will be needed to check it.
(2) Install a vacuum gauge as close to teh intake manifold as possible. Look for engine vacuum to be steady and around 20 in/hg. If the needle is bouncing, there is a valve leaking. if the needle is low, cam timing may be off.
Assuming the engine mechanical components test OK in teh abosteps, then it is time to try to determine why the idle is rough.
(3) Cylinder balance test the engine to determine which cylinder(s) are weak and causing he rough idle.
Hook up a tach to the engine and disable any idle control system, if it has one. Short the cylinders one at a time to kill spark, and note the RPM drop on each cylinder. if you find one not dropping RPM nearly as much as the rest, then that is the cylinder with the problem. Check the plugs, wires, cap, rotor for problems if you find any inconsistency in the RPM drop test.
Also, look for any engine air (vacuum) leaks. Take a can of spray carb cleaner and mist around the carb base and any components attached to vacuum lines. Pinch off the brake booster hose and see if the idle smooths out. Remove and check the EGR valve for leakage, creating an internal vacuum leak. When teh engine is running, try using a vacuum pump to open the EGR valve; the idle should get rough and the engine may even stall; that is the normal response. If not, then teh valve may be sticking open all the time causing the idle concerns.
(4) Assuming cylinder balance test results are even, then the last likely cause would be carburetion. Try backing the idle air bleed screws out slightly (maybe 1/2 turn) and see if the idle improves; if not, try running them in the same amount. If you are getting no change, remove the air horn (bowl cover) from teh carb and look for any sediment or dirt in the bowl that could be blocking fuel flow into one of the jets. if you find any unsuual sediment, try rebuilding the carb, especially making sure all internal passages through the casting are open.