Solenoid

It could be the 2nd idle solenoid in the above illustration. Check to be sure it is functioning by listening for a click each time the key is turned on.

No click? Check for voltage at one of the wires. If there's no voltage, then the problem may be between the solenoid and the switch.

Throttle plates

Did you remove the throttle plates during your rebuild? If they were installed upside down the plates will stay slightly cracked all the time and the engine will act like it's under throttle.