Leanness like you're describing can come from vacuum leaks in the rubber carb boots to the engine, very common after 35+ years. The K&N filter (if inside the stock airbox) will actually makes things a bit richer, overall.
That passageway from the tiny brass hole in the air horn to the main emulsifier tube: when this is blocked, or partially so, the mixture gets very rich and clumpy in "texture", not burning well. If yours is running lean, those are likely fine.
Things to look at:
1. If you recently replaced the fuel valves (float valves) with aftermarket ones, chances are 80% that the new ones have stiffer springs than the OEM ones did. This causes low float bowl levels and poor idle air screw control, and lean mixtures until about 1/2 throttle (or more). The only option, when using these stiff ones, is to raise the float levels: I typically go to 24mm, seems to help.
2. Did you check the float tangs on the floats where they press the fuel valve closed? If they have a tiny dimple there, it must be removed. This causes sticky valve action, both not filling and overfilling the bowls.
3. DON'T attempt to swap the hollow-tipped and solid-tipped air screws. There are other metering port changes that accompany those parts, and the mixed-up ones make for tuning headaches that you'd probably not like to take on.
4. A soft, 26 or 28 AWG mechanic's wire will snake into these passages OK. It takes a little patience, but you'll figure it out.
A word about our fuels: in these parts (U.S. West) the Shell and Phillips gas will make a white sparkplug electrode color and will burn lean. The Valero and Sinclair fuel makes the "normal" colors on the plugs. This is the majority of the gas we see. and it contains a MINIMUM of 10% ethanol, often up to 20%.
