I had the same trouble with a Saito 100 in my "Hype" with what seemed like gallons of fuel pouring from the engine on start-up.
I now only connect the tube to the carb just before starting (no cowel due to a change of engine type).
Sadly, my solution will not help your cowled Spit.
With the previous engine (OS FS90) with the carb at the same height, the problem did not occur so it must be this particular engine being susceptible to flooding that is the cause.