I bought a 1989 iroc-z 5.7 a couple weeks ago. It had been sitting up with out being started for over 3 years. I put fresh fuel in . It will start but has a miss and doesnt have the power i feel it should have and after it runs for about 20 or 30 minutes if you turn it off it wont start back till the next day. I mean it will turn over fine but will not even try to fire up . It is getting spark to the plugs ( new spark plugs ) and fuel to the fuel rail and i can hear the fuel pump kick in . Any ideas before i start replacing stuff. Im not a great mechanic so thought i would asked for ideas before i did much more .

 

Best bet, start with the basics. First thing I would of done, since the car sat for 3 years is unplug all the fuel lines, and blow them out with compressed air. Second, i'd drop the tank and run kerosene or even gasoline through it just to flush all the varnish out.
From the sound of it you could either be dealing with a fuel delivery problem, (not enough pressure/bad pump, or to much **** in the lines, clogged injectors or just plain old bad gas being ran to the engine. If its just bad gas, just let her run till she almost clears the tank, you might get lucky. Now besides, fuel, easiest thing you could do is check for spark on all 8 spark plug 'wires'. See if you come out with a nice fat blue spark. Wouldn't be surprised if its just a tune up you need. Let me know what happens partner