Well, new carbs are not cheap. Can you hold the throttle shaft and get it to settle down?

No, not at all.

"when I start it, it goes into a high rev and then kills itself"

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more information is needed to determine what is going on. I assume you moved the throttle control to full power when you attempted to start. There may be a limited amount of fuel in the carburetor float bowl and this is immediately consumed. Is this a gravity feed system?

On opposed Twins, the fuel pump is built into the carburetor. Evaluate the entire fuel line from the fuel tank to the carburetor. Poor clamp connections and cracks in the fuel line may allow air to enter. Poor clamp connections on the pulse line may affect fuel pump performance. Dirt or blockage will affect fuel delivery. A plugged vent in the fuel cap will affect fuel delivery. The carburetor may have internal blockage from the effects of ethanol fuel. or dirt.