It is not just one bad spark plug or the engine would misfire (steady beat). I don't think that spark plugs are causing the problem.
I think that you might have a leak in a vacuum hose (most likely cause of the problem, especially since replacement of the valve cover could have pulled off a vacuum hose or broken one). You might, instead, have a broken vacuum device (like a choke pull off that broke its diaphragm).
There are many other causes for weak acceleration. The timing might be wrong. The carburetor might be misadjusted. Etc.
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