Some fuel oil spitback into the air silencer is normal.
If you remove the air silencer and place your hand near the front of the carbs one at a time see if there is one spitting more than the others.
It sounds like you might have a reed problem or reed plate gasket issue.
Since the reeds and intake tracts are all separate you will find one that spits alot more than the others if there is a reed problem,
Marine Mechanic
Evinrude Master Technician with 27 years of experience.