You have to think outside the box here. Each intake stoke draws air and fuel into the bottom end of the engine. On the piston's down stroke pressure builds in the lower end of the engine which drives the fresh mixture up into the combustion chamber, but positive pressure in the bottom end of the engine also tries to push some of the fresh "FUEL/air" mixture back out then carb throat at low rev's because there is no vaccume created in the header from the tuned pipes reverse pulse to help draw the fresh charge into the combustion chamber.

Some of the excess mixture goes into the boost bottle. So you end up with air and fuel in the bottle. Since you can't see air you are only noticing the fuel in the bottle but there is air in there too.

Which btw, if you are finding much fuel in the bottle then you have it mounted wrong. It really should be mounted upright with the outlet pointing down and it should be above crank height.


(p.s. don't talk to "hobbyshop" guys.)