I think thats the cold start valve. Take the fuel line off and plug it too be sure it is not leaking.
You may have gone too far with the t-stat. It was probably so loaded up that it took a while to appear lean. Just a thought since you got this down now.
While you are eliminating other things (cold start valve leaks) disconnect all the electrical to solenoids on the rear, I have seen the starting enrichment solenoid be hooked up improperly and throw the rack rich. When warm none of the solenoids are necessary for proper running.
Bosch Master, ASE Master, L1
33 years MB technician