The fuel tamk would not pressurize if the lines were reversed. I have 3 of the 91ACs and they are as easy to run as te rest of my YS engines. If takes a lot to get it started but runs wonderfully after it is running I would look at the easy to eliminate suspects. Make sure it has a YS #5 or an OS #F plug in it and the glow driver is charged and not weak.

If your carb is dripping fuel you might want to take a peek in the regulator for debris and gelled fuel.