I am having trouble locating a vibration. It happens when I start the engine in neutral (c-4) ,at idle, as I rev the engine up, when driving the Ranchero down the road, gets worse at around 2,800rpm, gets smother at about 3,000 rpm. I have changed the flex plate to an SFI approved 50oz model, changed the harmonic balancer with a similar stock model, checked the spark plugs, wires and injectors for electrical problems. Still the vibration is there. I have run out of answers and hope some of you would have an idea...
1965 Falcon Ranchero, 1990 5.0 HO EFI, C-4.