This might be caused by vibration at certain rev bands if your carbs are hard mounted. Frothy fuel will provide a weaker mixture when resonance occurs but this probably isn't happening at tick over in your case.Quite a lot of weight cantilevered out from the block unless you have steady bar underneath like on the Volvo injection manifold. Will be worse if you have the long inlet manifolds. This leverage will also act on the manifold to head gasket. Some good sealing soft mounts might help or the traditional fat rubber rings or mounts and Thackery washers. Example of modern ones here
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