Riding Mower only starts with starting fluid(ether)?

I have an old model Ariens riding mower. it has seen better days. i purchased it from a repair shop for next to nothing. I completely redid the fuel system, completely flushed and changed the oil system, as well as other misc. repairs: fixing the tires, sharpening the blade, tightening everything, etc. the only thing i haven't been able to figure out is why i can only start the mower by spraying starter fluid into the air filter. after spraying i can either pull start or turn the key and it will start right up. i was wondering is there anything i can do to make it start up without pulling apart the air filter every time. all suggestions will be appreciated.