Carburetor : vnb1t 1961 125 standard

hello to everyone

i have a  noticed that the engine casing underneath the carb box (rear) is constantly wet with fuel/oil

so i renewed the gaskets but this has made no difference . there are no leaks that i can see ,  when run without the air filter a lot of fuel spits out of the carb when the throttle is opend . is this normal ? the bike runs ok (a little under power) and the spark plug is chocolate brown

the exsisting carb is an si 20/15b (choke in air filter) not sure on the jet numbers 

i have seen 125's running 20/20's and bigger

i was thinking of upgrading to a larger carb what would your recommendations be 

any advice much apprieciated

ace shop by the way!!!