1978 Datsun 620 P/U, Idle problem

I recently replaced my carburator witha Holley that appeared axactly as the factory unit. When I started the truck the idle was extremelt high. I haven't been able to get it to go back down. I even put the old carb back on, but that didn't work either. I am almost possitive that there are no crossed or forgotten vaccum lines. I have tried the idle adjustment screw on the carb, but it has no effect. Even the local mechanic couldn't figure it out. If anyone has any idea what is going on please help me out.