i have a junkyard motor in my blazer and it has a rough idle but no cel. should it be smooth at idle as i know it has a balance shaft. im worried the valve guides are bad as the last motor had oval shaped guides when i tore them down. it has new plugs and wires. new cpi spider and pump. and a new cap. unknown on the rotor as it was with the junkyard motor. is this 4.3 known for vibratioor what ns

 

It should idle pretty smooth. You may have a worn distributor gear, stretched timing chain or other issues. Hard to say without doing complete diagnostics. No error codes doesn't necessarily mean everything is working right.