B&S only runs on gas poured in carb

it might be the tank. now if it has a hole where a hole shouldn't be toss it. you need to either find a good used one or go to a small engine repair shop and either get a new or find a used one there. now if it looks fine but is rusted inside or under the carb seal if it looks repairable then fix it. carbs are alluminum and the tank is steel. just like they say opposite metals don't attract and they will corrode or rust the tank under the seal if it sat in the rain alot. now if the steel is pitted around or under the seal don't bother fixing with silicone or anything else it don't work it cause's more rust on the rust. now if the engine is lets say more than 5 years old go ahead and replace your diaprahm and carb gaskets. if the tank is repairable clean up the rust and paint it with paint made to withstand gas. now on an odd chance both the carbs need new diaprahms just check em first. if they look worn, torn or you can see tiny little dots of light shining through replace them first. they are cheap and most likely the problem. i had one that would do what you were saying and i replaced the diaprahm and problem solved.