95 runs bad, no codes

Glad to find you guys, I restore musclecars here in L.A. and computers scare me. So... '95 silverado new 4.3, auto, idles at 550 rpm ruff, sometimes stumbles at 1200 rpm real bad, feel a miss at cruise speeds and showing NO codes. Bad ECM? ran like this before the new engine, only part not new is throttle body. did not throw code after I disconnected wire to set timing.
had the pipe sniffed and it shows lean condition.
thanks!

Test for fuel pressure.