There's a temperature sensor threaded into the underside of the intake manifold that signals the engine control unit to run rich on a cold start. It works in conjunction with the air regulator (big black tube off the side of the throttle body) mounted on the side of the engine block under the intake manifold. MAKE SURE your block and head are filled with coolant. It's possible the sensor isn't in contact with coolant so isn't sending the signal. For me the best way is to unthread the air bypass valve from the top of the thermostat housing, pop the cap on the radiator and fill the radiator 'til coolant flows out of the thermostat housing. Some like to park on a hill with the engine up and run the engine with the radiator cap off to burp any air out. The important thing on any engine rebuild or head rebuild is to make sure no air is trapped in the galleries of the head. I didn't on one head rebuild and was rewarded a year later with melted exhaust valve seats.