When to repair a spark plug and when to replace it

Spark plugs aren't very expensive, but over time buying new plugs can add up.  A new spark plug will often help your engine run better, and can fix some starting problems, but there are times when you can repair your current plug rather than replace it.

If your plug isn't very old, but it's black and dirty, you can probably just clean it off with a wire brush and lint-free rag.  Once all the carbon and other deposits are cleaned off, it should spark better.

Over time, the gap can get out of adjustment, either from the electrodes wearing down or just from general use.  Regapping the spark plug will increase its efficiency, and it only takes a few minutes.  You can find a guide to regapping your spark plug here:

How to gap a spark plug

It's generally a good idea to check the gap, and regap if necessary, any time you remove the spark plug, especially if you're cleaning it.  Keeping the gap adjusted properly will make your engine run better.  Many push mower spark plugs require a gap of 0.030 inches, but you should check your engine manual to be sure.

A wire spark plug gauge is more accurate for old spark plugs, because the electrodes don't wear evenly so they won't remain flat over time.  For newer plugs, either a wire gauge or a flat or ramped gauge will work fine.

If the spark plug just looks old and worn-out, even after you've cleaned it, you can't hurt anything by replacing it.  You should also replace the plug if the side electrode bends very easily -- even if you gap it correctly, it may not stay that way for long.  If you're having ignition problems, even after cleaning and gapping the spark plug, a new plug may be in order.