The overflow line is not needed for MOST installations.

The reason to use the 3rd line (with a plug for while the engine is running, or you won't have muffler pressure in the tank) is a lack of access to use the carb line to fill the tank.

If you can conveniently pull the fuel line from the needle valve and fill the tank, you don't use the 3rd line. There are very few sport models that the .15 to .25 engine class would go on where you can't get at the needle for this purpose.

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typos R us... [:'(]