The harmonic balancer consists of three parts:
An inner hub that mounts on the crankshaft
An outer inertia ring - helps dampen vibration ( also has a timing mark stamped on it)
A rubber isolating ring that bonds the inner hub and inertia ring together:
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Sometimes (very rarely) the bond between the rubber ring and either the inner hub or outer ring fails and the inner and outer move in relation to each other. When that happens, the relationship of the timing mark to the position of the crankshaft is altered and can no longer be used to time the engine.
But, I wouldn't be too worried about your harmonic balancer right now.
Here is your original description of the problem:
But it runs horribly. I don't think the plugs or wires have ever been changed but even at 140 degrees f it sputters and dies going into any gear
but once it reaches 175+ it's just fine.
I would concentrate on getting your choke set correctly. And don't jump to the conclusion that you need to rebuild your carburetor just yet - q-jets are very dependable.