If it only runs with the choke on then it is not getting enough fuel
That leaves you with two options
1) carb is not putting enough fuel into the charge
2) air is entering after the fuel has been mixed into the charge
To check for the latter start the blower then saturate the area between the carb and the block with WD 40 or similar from a trigger pack, not a Spray can ( fire risk ) .
If the blower stops or blows white smoke then there is an induction leak.
If that makes no difference then after running the blower for a few minutes, either pull the fuel line off the carb or open the fuel cap.
The fuel tank is pressurised by the return line form the pump
If the tank is not holding pressure not enough fuel will get to the carb.
The pump does not suck very well, like most pumps it blows and the blowing increases the pressure in the fuel tank which forces the fuel up to the carb.
This is paticularly important in back packs because the carb is a lot higher than the tank so the fuel has a long way to go against gravity .