YOU ARE CREATING A VACUUM BY BLOCKING OFF THE AIR INTAKE, IT IS SUCKING THE GAS OUT OF THE IDLE JET OR THE CHOKE. The low speed jet is either the wrong size or clogged up, You can try squirting in some carb cleaner as it is trying to run, or just take the thing apart again, and make sure the jets are clean, get a guitar string wire in there and blow through the hole. And before you do that, check the pump, make sure it is pumping fuel, not just sucking it in. Check it out.