carby leaking

Hi Guys hoping for some advice.

After my previous bike was stolen my dad and I fixed up one of his spares. We had to buy a new carb.. so we did. Problem with new carb is;
I need to have engine off, then turn fuel on wait for it to over flow from carb (whicj is part of issue) then turn fuel off and off i go until fuel runs out and engine shuts off then repeat.If i turn fuel on when engines on it stalls and dies almost immediately.

Any idea what can be done. My Dads currently taken apart another carby and cleaned it to try tomorrow. But just after any advice as to what to look for if we still have issues as this is a brand new carby.

Cheers

If it's a CNS carb throw it away and get and NT....if its an NT take the bowl off and bend the brass tang (that's under the float )down a bit....