The fuel line not only extends your reach, it also prevents the plug from becoming cross-threaded.Lots of motorcycle engines have spark plugs buried in a cavity deep inside the cylinder head. Unless you've got fingers like Gollum, this can make threading a fresh plug into its seat somewhat challenging.
One solution is to stick the plug into the appropriate socket and try to start it that way. The only thing wrong with that approach is that it's hard to feel what's going on down there. Consequently, the plug may end up becoming cross-threaded, possibly stripping the threads, especially if you feel the plug's resistance to turning can be overcome with a little brute force.
A better method is to force a piece of fuel line over the spark-plug insulator as shown. Use the fuel line to start the plug. If the plug starts to cross-thread, the fuel line will slip before any damage can occur. You can then back the plug out and try again until the plug engages the threads correctly.
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For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.