I have a 66 200 6 cyl. that feels out of balance at 2000-2500 rpm. Is poor balancing common with these older engines? Are these engines internally (neutral) or externally balanced? Thanx for your input....Dave
Internally balanced, although a bad harmonic balancer, missing fan blade, engine misfire etc would cause a problem. Because of their long crankshaft, straight sixes face more balance issues than V-8s, but you shouldn't be feeling it at 2000-2500 rpm.
Does it feel out of balance while the car is moving or are you seeing this when you rev the car while standing still. My gut says check the driveshaft balance.