Chevrolet Express Engine Misfire

I Replaced Engine With A 95000 Mile Used Engine. After Replacing Engine There Is A Misfire At Mid Throttle. Engine Runs Great At Idle And...

Replaced The Engine With Used Low Mileage Used Engine. Thought Valves Were Sticking. Newer Engine Runs Just As Bad, So I've Eliminated...

At 45 To 50 Mph I Let Of The Gas And Slowly Put My Foot Back On The Gas It Kicks And Fills Like It Skipping Until I Go Heavy On The Gas . It...