I've got a sneaky feeling the head is cracked in the area of the guide for the valve that got battered. It won't take much of a leak to affect the mixture of both cylinders fed by that port. Be sure the valves are fully closed and use a Mighty-Vac or even a vacuum pump used for evacuating an air conditioner. Obviously, tap into the port adapter with a nipple to accept the hose from a vacuum gauge.

I'd suggest using a rubber freeze plug with the center bolt drilled and tapped for a vacuum pump nipple. You can pick a plug that's close to the diameter of the intake port.
Jerry


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