The carburetor bowl drain overflow leaks gas when the petcock is on reserve but not when it is on the regular on position. Petcock problem? Vent problem?

 

all I can think of is a little more head pressure from a slightly taller column of fuel. in any case, sounds like time for a teardown & a new needle

 

To me if the over flow is flowing your float isnt sealing.

 

I agree, the different position petcock problem didn't make sense so I put more gas in the tank and it came out the overflow on both positions. I had the carb out for jet cleaning but neglected the float and the float valve so its coming out again for a more through cleaning.

 

there ya go...

double check the float to see if it has liquid in it. buy a new needle. inspect the seat carefully. if its boogered there are things you can try (with varying degrees of success). the good news is it's almost always the needle