I finished building a Zombie XP farm the other day. It works on the principle water pushing them to a 2x3 passage in the center of the wall, taking them up via a water elevator and then letting them fall. It worked fine from about 30 min but then stopped. After 10 more min, I opened a hole to see what had happened. There were dozens of Zombies waiting to enter the elevator. I put the game on peaceful to check it out. All that I found was a chicken swimming on the 1st water block. I guess chickens and maybe other thing can spawn in a dungeon? Can a chicken block Zombies?
it's most likely the zombies blocking themselves. It's most likely a design fault... The classical zombie/skeleton spawner designs can do that sometimes. You could maybe provide a few pictures so we can see what's wrong with it. Also, I don't believe that it's because of the chicken. If there's a chicken it's probably from a chicken jockey, I'm not sure whether spawners can spawn those though.
Pictures would help (particularly of the water flow pattern) and the water elevator.
First thought is that the water flow creates a situation where multiple zombies try to enter a passage at once (althoug a two wide passage ought to make that quite unlikely).
The chicken may have generated as a mount for a chicken jockey (a chuckie/babby zombie mounted on a chaicken), but I'm under the impression these chickens follow zombie despawn rules, but perhaps the switch to peaceful removed the rider and insufficient time has elasped for it to despawn?
[Or perhaps switching to peaceful converts a jockey mount to a normal chicken? IDK]
One block high mobs require special work-arounds for bubble based mob elevators [bubble-vators] which may be an issue if you are not using a column of water blocks. (Again, though, a 2-wide passage should be nearly impossible to jam…)
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It's probably what the above stated. Too many zombies colliding with each other prevent them from entering your shaft, and to go along with that if baby zombies got in there they won't use your ladder system to get up and prevent other mobs from doing the same.
You could try making a 1 block high cutout, and a 2 block drop behind your water ladder. Set up so small zombies get pushed passed the water ladder and out of the way of the rest of the zombies.
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I prefer a center flush for this very reason. The popular design with stepped floors and diagonal flushing tends to push all the mobs to the door at once, and they bump shoulders before going through, until too many bump shoulders and get stuck.
With a center flush the mobs form a nice single file line and exit the room in an orderly fashion.
Oh, and the chicken was left over from a baby zombie chicken jockey. The baby probably dismounted when they hit the water so you only saw the chicken. Fortunately those chickens follow hostile mob spawn rules and will despawn.
Chickens jockeys can clogge the water flow , it happend to me many times ,its the only thing i can't figure out how to kill the chickens before they reach that point. but in other matters i made 2 small holes.
one under the water flow , and filled it with lava so the zombie kids fall into lava because they can't reach the other part , or 2nd just in case i made a water flow with 1 block gap so that the zombie kids will flow right into a second lava trap
Thanks guys. It happened again so I put the game in Peaceful and checked. It was not a design problem; it was an execution of the design problem. The last time (when the chicken was there) I went through the building of the water elevator again. I then proceeded to put a sign in the work place creating a 2 block air pocket. That was the problem this second time. After fixing that issue, I have had no issues. I did add a 1x1 block water tunnel to push the baby zombies into lava. I made the tunnel long so that light from the lava would not be an issue. I also changed the kill area. With the relatively new limit to how many entities can occupy a block, I needed to make a change so that I could AFK longer. I put four hoppers in a row feeding into 2 double chests. I timed it and now it takes @8:45 min before the zombies start suffocating.