mbk ovetto 50 cc?

i have a problem with the carburettor over flowing out of the hole on the bottom of the carb .the bikes revs wouldnt settle so i stripped the carb to find a bronze or brass pin inside the chamber had pushed out so therefore the slider wasnt located properly,when i put the carb back on it has made a big improvement but has started leaking fuel out of the bottom of the float chamber i havnt messed with the float chamber just the throttle slid also what is the secondry pipe for below the fuel line when it comes out at the bottom of the tank it has two lines one for the fuel and another which im unsure about why they both go to the carb