Luke warm heat 1996 Corsica

Rebuilt the engine and installed new heater core at 82,000 miles ago and it take about 20-30 miles before i get luke warm heat, i installed a TECH2 scan tool and the engine is running at operating temp, the heater hoses are hot from the heater core. if i turn the temp knob just a little bit down, the air is real cold. any ideas? Thank you