A spark plug is used in an internal combustion engine to ignite the fuel in order to cause motion. In a car, there will usually be at least four, but in the humble petrol strimmer, or string trimmer, there's just one. And as with anything man-made (or anything else really), they do eventually wear out and will need replacing.
If you've invested in a petrol strimmer, you really should learn how to maintain it. It can be really quite simple to perform most maintenance tasks, but some people never learn. When something does go wrong, they either pay the shop to repair it or just stop using it and leave it to gather dust.
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If you're really not at all confident at performing basic maintenance tasks, you should think carefully about whether you should be buying the petrol variety of strimmer in the first place. Although I thoroughly recommend using the petrol variety where possible (for the obvious reasons of power, efficiency, etc.), if you aren't comfortable with it, there's nothing wrong with deciding to buy the electric variety. Although they're not as powerful to use and have other limitations, the maintenance of electric strimmers boils down to simple cleaning or sometimes changing a fuse.
Ok, so with that out of the way, here's how to change the spark plug. Although all strimmers are different, with the majority of models, these general instructions are basically all you should need to do:
1. Pull the spark plug hatch off;
2. Use a 3/4 inch deep well socket to remove the spark plug;
3. Pull the spark plug out;
4. This step is optional: If you own a compression tester, at this stage you can check the compression (if you don't own one, skip this step). Insert the tester and make sure it fits tightly. Pull the crank four times in the full throttle position. Check the compression - generally anything over 90 to 95 pounds of pressure should enable the strimmer to run;
5. Put the new spark plug in;
6. Tighten it with the 3/4 inch deep well socket;
7. Put the hatch back on.
And that's all there is to it.
Having said this, if you have made the investment for a petrol strimmer, and you're not comfortable changing the spark plug, any power tool dealer should be able to help you. The spark plug is cheap, and the job very quick, so it shouldn't be very expensive to have a professional do the job.