As Jim says, there are various items that influence the fuel mixture. Check the CO level at operating temperature first and foremost; it should be around 3-4% at idle. Easily adjusted with the screw at the back of the fuel injection pump (clockwise to richen, counterclockwise to lean out the mixture) in combination with the large air intake screw on the intake manifold.
This affects the mixture at idle, not at mid- or full-range though. That is influenced by the number of shims underneath the barometric pressure compensator widget sitting on top of the fuel injection pump.
When those two adjustments don't get you to the desired mixture level, chances are the WRD (Warm Running Device) and/or CSV (Cold Start Valve) are not operating properly. You really should diagnose these two first, along with taking the linkage tour as Jim suggests. This is important because how the gas linkages are set up has a significant impact on how the engine runs. Sloppiness or loose connections there need to be corrected. One of the items in that tour is the butterfly valve in the air intake ("throttle body"), that at idle should be closed, to the point of almost binding. ignition timing, and the condition of the distributor, also are of importance in how the engine runs.
It all sounds pretty complicated probably, but with all of the detailed info on this site you will work it all out ...
