Engine Revs Fast After Warm - Possible Causes?

[jb3t]

My 1978 C10 Big 10 runs very well. Starts right up, idles well, and runs great. However, I have noticed that after driving it for a few minutes, the engine appears to rev. Example, I will pull up to a stop sign and it still wants to go. If I slip the truck in neutral, it will rev pretty high (no tach so I don't know RPM). If I pump the gas while in N, it may settle down. It's a bit of a safety issue that I need to figure out. It's a pretty high rev. Like I'm pressing on the gas and break at the same time.

Any idea what may cause this? Hopefully its a simple carb fix.