'88 Dodge Dakota running very rich after engine work

Okay: the first time this happened was after replacing the head gasket (driver’s side only) and fixing a failed rocker arm. After hooking everything back up again, truck runs VERY rich (to the point of misfire). Plugs, when pulled, look as if they were covered in coal dust.

It would run like this until up to operating temperature, then run normally.

The second time: had to take the valve cover off again, as I was sold the wrong rocker arm replacement. All I did was pull the valve cover and the rocker arm assembly, replace the rocker arm, and re-assemble–no sensors were disturbed.

Again, ran rich to point of misfire, until thermostat opened (roughly), then normal running since.

What’s going on? This doesn’t even seem to be “open loop” running, but rather “emergency limp-home rich” running. Could the engine sense a lack of coolant for whatever reason? Why would popping the valve cover have any effect?