I was tinkering with the carburetors and noticed the vacuum line going from the fuel valve to the carb. What for? I understand that carbs mix fuel and air, but where does the vacuum come into play? Just wondering
-Ryan

The fuel selector switch has no old-style OFF position.  Instead, the ON and REServe positions will only allow fuel to pass through to the carbs when the engine is running.  How can it tell the engine is running?  By a vacuum pulse from the carbs.

The PRIme position allows fuel to pass through no matter what; it's a gravity-fed route that is useful for refilling the float bowls after the carbs have been worked on, etc.  (The engine can't run with no fuel, but the ON and REServe positions won't pass fuel unless the engine is running.  PRime is the way out of that particular Catch-22....)

cool.