I've just finished doing the same job on the same engine 30 mins ago!

I think one of two things has happened, - a vacuum hose has been left off the inlet manifold and this is letting air past the throttle body. Alternatively something is holding the throttle partially open, - did you refit the accelerator cable bracket correctly? If so make sure that the accelerator cable is correctly threaded around the quadrant on the throttle housing.

Cheers
Mark