I'd suggest you to mount the engine on a test stand , start her & tune it,
prior to installing her on your firewall,
ofcourse
UPSIDE-DOWN...
You won't get any trouble at all...
I have also
x2 4strokes ,
SAITO FA-120S
& FA-150S
both installed upside-down....
they run awesome!

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[youtube]http://www.youtube.com/watch?v=ejTNK5kiaGQ&feature=mfu_in_order&list=UL[/youtube]
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[youtube]http://www.youtube.com/watch?v=6xjQlowDuIc[/youtube]