It's been my observation that fuel lines are oriented in whatever direction is most practical for the boat in question.
Primer bulbs mounted vertically with the flow direction in the "up" position will probably drain slower due to gravity helping keep the flapper valves closed. If you have faulty primer bulb you can make it function by holding it with the flow in the up direction for the same reason. I don't think I've ever seen one 'mounted' though. Make sure you leave enough room to grab it without banging your knuckles!
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