When do spark plugs go bad?

[LORDY611]

I have this 94 Merc 40 that supposedly has seen very little use. Looking at everything, I must agree. The plugs look original because there is some black overspray around the base that contacts the head. These are the flat electrode type, so I cant see wear like I could if the conventional type electrode was there. So the question is, can this set of plugs be bad just due to just being 13 years old? I know that conventional wisdom would tell me to replace them because they are not too expensive. But do I really need to?
click on these to enlarge.
Motor overall

spark plug closeup

 

[Silvertip]

Re: When do spark plugs go bad?

I think you already know the answer to that question. Of course replace them. They are 13 years old. The overspray may have been the result of someone trying to "touch up" the engine before the sale. The plugs are called surface gap plugs and can foul in the same manner as conventional plugs. While you are at it, the fuel filter, thermostat, bypass valve and water pump impeller are also long overdue for replacement if they are original. This preventive maintenance will ensure at least those wear items are in good shape. After 13 years the water pump vanes will have become hard, can disintigrate and cause the engine to overheat. The thermostat will stick shut and the bybass valve will not open at high speed. All of this creates the overheat potential that can be easily avoided. Lastly, consider immediate carb rebuilds. Gunked up carbs are responsible for more engine destruction than you can imagine.