That spring loaded "vent" is actually the tickler. You can push it down which opens the float valve and lets fuel in when you first start the bike on a cold morning.
Fuel should not come out it like that, that shows its "flooding"
You need to pull some of the carby apart to clean it all.
First remove the small bolt on top of the part the fuel line attaches to, you will then be able to lift that part off and there will "should" be a plastic guaze filter under it, clean the filter.
Next remove the three screws on the side of the carby and take the plate on the side off. Behind this you will see the float and a needle sitting on top of it, you need to remove the float and needle and blow air back up through where the needle goes, thats where the needle "seats". Once you have cleaned all this put it all back together and try it again. Be carefull not to lose the small brass spacer that goes on the outside of the shaft the float pivots on. Take carefull notice of how it all comes apart so you can reassemble it all the same way.
All this is only if you just want to get the bike to run. If you intend to ride the bike then you need to remove the carby and dismantle it and clean it properly and fit new gaskets etc and probably a new "needle". The needle in it is probably a solid brass one or maybe plastic, you can get brass ones now that have a rubber "Viton" tip and work much better.
