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As Goatman suggests, you may have extra air sneaking in thru a cracked vacuum hose or intake gasket. The ECU is setting the cold mixture based on info from the ECT (Engine Coolant Temp) sensor, but the extra air will "dilute" that rich mixture.
I use Propane to test for vacuum leaks. It's less messy—and no more volatile— than carb cleaner or other sprays. Take the nozzle tip off a propane torch and replace it with some snug fitting rubber hose about 2 feet long. Practice with the valve to get a moderate gas flow (not a roaring blast).
With the engine at a warm idle, open the gas valve and poke the end of the hose around each injector for a couple of seconds. If a seal is leaking air, you may hear an RPM change when the propane gets sucked in and burned**.
Do the same around any other suspected areas, like hidden vacuum hose ends and don't forget the intake manifold gasket itself.
**If you use a Digital Volt Meter to "read" the O2 sensor output voltage (O2 signal wire disconnected from ECU), you'll see the signal voltage rise instantly if any propane gets drawn in.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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