With K-jet, if the main pump's misbehaving significantly it will upset the mixture. Ironically, slightly low pressure will cause a rich mixture, and slightly high pressure will cause it to run lean.

A dirty or failing control pressure regulator (cpr) can also cause mixture problems -- and is more likely than a misbehaving fuel pump. The cpr's different from the fuel pressure regulator, which is in the fuel distributor. The cpr controls the fuel pressure during a cold start and warm-up.

You can assess the system pressure, control pressure, and rest pressure using the appropriate fuel pressure test gauge setup, which is designed specifically for this. It attaches to the cpr. I believe Harbor Freight sells a gauge set that either fits directly or can be easily adapted. The "official" Bosch or Volvo gauge set is expensive (what a surprise!).

Also check the connections at the frequency valve and its relay.

Be sure the throttle and throttle switch are properly adjusted. The switch MUST click when the throttle returns to idle -- it controls the idle speed system.
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Don Foster (near Cape Cod, MA)