What are the potential causes other that the adjustment on the throtle body, that causes an engine to lean out?

 

Bad O2 sensor, if it was telling the ECM that the exhaust was too rich, then it could lean out the mixture.

Bad coolant temp sensor, telling the ECM that the engine is hot when it's cold, causing it to run lean when cold.

Just some.....

 

hole in intake that allows more air in after the MAF/VAF

 

fuel pump, fuel filter, fuel pressure regulator, injectors, AFM calibration, vacuum leaks, low battery voltage