Jonathan is right about a proper fill and calculation for the amount of gas used. An eyeball guess is never going to work well enough. If memory serves me the tank is closer to 3 gallons U.S.
I have to wonder about the spark plug plugging a vacuum line. Seems like a goofy way to accomplish something. I suppose it works for the tech but I would opt for screwing in a small bolt finger tight or just slip in a golf T.
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