Assuming your tank is properly installed, and the vent is open, this is very normal if the prop is turned over slowly. It's a function of the intake timing. The timing is optimized for high-rpm running, and as such the intake rotary valve stays open beyond TDC, usually for 20-50 degrees (can't remember exactly what's typical). That means that if you rotate the engine slowly, first the engine will suck, then it will blow a bit from the carb. Some engines actually spit a bit of fuel from the carb at very low idle.

To prime the engine, flip it as quickly as if you were trying to hand-start it.

Some engines will also be nearly impossible to prime when hot, because as soon as some raw fuel hits the hot engine, it flashes to vapor and blows the fuel back to the tank, emptying the fuel line. My Norvel .061 is a great example of this, tho any hot engine may exhibit this.... 'course, there's not alot of reason to prime a hot engine anyway!

J