Hy jim54521, please remember to tell us what year, engine and size of truck you're asking about. I think the hissing sound is the sound of a vacuum leak, and the engine will never run correctly with a vacuum leak. It sounds like the accelerator pump is working correctly, by giving you a squirt every time you pump the carb linkage. I think finding/repairing the vacuum leak should be your first priority. If when you have the truck running you place an unlit but turned on propane torch in the area that you hear the hissing, if there is a vacuum leak the engine will speed up when the vacuum leak draws the propane in, hope that helps.
