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I've inherited a 1979 Yamaha xs400 that needs some attention. Whilst preparing to remove the engine I came across this tube that protrudes from where one of the 2 carbs meets the block. The 2nd carb has a nozzle where a 2nd tube could go, but this has been blocked off with a cap.

Would anyone know what this is for?

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Well... Mr google said:

The OEM petcock is vacuum actuated. With a properly operating petcock, unless the engine is producing vacuum the petcock will not flow gasoline while in the ON or RES positions. When in the PRI position gas will flow even when the engine is not runing. The tube the shop is talking about is a vacuum tube running from the left carb intake boot to the petcock. It is not unusual for the vacuum actuated portion of the petcock to fail and the owner to clip and plug the vacuum line going to the intake boot to eliminate the associated vacuum leak.

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Hope that helps.

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