Andy wrote:When it won't fire properly!

Its true, and that is the only way to tell.

I've never run the wrong plugs (other than running shorty RS plugs) but I've run some in pretty awful condition (bet it shocks some of you to find me cutting corners eh?) and I've never found had a problem with one.

Though to be fair if/when I do have a problem, new sparkplugs are the very first thing I try. They really help with starting if nothing else.
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