Not had one of them in bots before, so apologies if this is more of a question than an answer....
The spring might be there to allow the fuel to "slop" slightly when bouncing through waves (i.e allow the valve to be more constantly open or closed as the float bobs up & down) Any Pics? Is it between float arm and valve pin, or acting directly on the pin? Also what is its "extended" setting - valve closed or valve open?
The float valve is basically there to stop too much fuel getting into the carb bowl. Once the fuel is in the bowl it's sooked up by the vauum in the carb passage, so a reasonably constant level in hthe bowl is needed. The fuel pump only moves the fuel to the carb bowls, not into the engine airflow, coz that's called fuel injection! If your float is too low, the slightest amount of fuel in the bowl and the valve closes, so the engine is starved. Likewise float set too high & the engine "drowns" at idle as the fuel is literally pumped into the engine at far greater quantities than the engine can dispose oif at idle.