revs to 2500 RPM when started

[F350none]

I just bought a 02 F-250 4x4 with 490K miles non-running for $1100. I put a fuel pump (the one on the frame under the driver) in it and got it running. When I start it it runs rough until you play with the throttle a little then it idles fine (Been 20 to 30 degrees out). Ran rough going down the road but has gotten better. Today it started revving to about 2500RPM for a couple seconds when started then it calms down (35 degrees out today). Only other thing I know is it sat for at least 4 months. It had a half a tank of fuel. I topped it off and added some Howes. I have also changed the fuel filter. Any ideas on what is causing it to rev like that. Dont want to blow the engine.