Eng S/N 0B776858, 1988 4.3L LX. Symptom: Runs on lake at 3/4 throttle all day. When I try full throttle it tries to accelerate then falls flat. Bring back to 3/4 its happy. Starts and idles great. I have rebuilt the carburetor. I think the fuel pump is original. I want to replace it just to...