3 line:

One to muffler for pressure

One to carb

One to fuel

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A 3 line is much easier with a cowled in situation as you can have the line hanging out the bottom of the cowl, or in a fuel dot somewhere. It's plugged on the end, when you want to refuel, instead of taking the cqarb line off, simply unplug that line, use it to fuel, then replace plug and fly