Fresh power valves would be a good maintenance item as mentioned. While you have the carb opened up, please note the power valve vacuum opening numbers and the jet sizes installed front and rear. The power valve numbers are hard to read and are on the metal part of the valve. If you see a number like 45, that means the power valve opens at 4.5" of manifold vacuum. If it says 65, that means the valve opens at 6.5" of vacuum and would deliver extra fuel sooner. If it's leaking as mentioned, the engine is seeing that extra fuel all the time. As far as the spark plugs go, a B5 spark plug would be one step hotter than your current spark plug. NGK numbers are opposite of some other brands for heat range. Higher numbers are colder plugs. Can get confusing.
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