My flushing dark room mob trap works fine in 1.4.5. You need to determine whether the problem is mobs not spawning at all, or whether they are spawning but not moving into your trap.
P.S. Do mobs damaged by the environment give less exp than those you kill with full hp?
No, as long as its not the environmental damage that kills them you receive full XP. Most XP farms take advantage of this by using environmental damage such as a fall or suffocation to reduce the mobs health to low levels to make killing them faster.
In terms of gaining XP 30 levels in 15 minutes is not too bad. If you are using a properly designed farm you can go AFK (Away From Keyboard) while the mobs build.
3-4 hours branch mining should have netted you quite a few diamonds. Are you sure you are mining at the correct layer (I mine layers 11-14)? Are you using a proper branching technique (I use a long central shaft with branches spaced every two blocks)? I just did a quick test and 30 minutes mining time using stone pickaxes yielded 9 diamond ore. This is in line with my experience in previous worlds.
A skeleton XP farm is best to begin as you get bonemeal and arrows as well as XP. Once you are more established travel to the Nether and establish a Blaze XP farm.
Simply branch mining in a straight line at levels 11-14 is enough without having to do any fancy tricks or bypass dirt/gravel. I normally use a larger main tunnel with 2x1 branches either side. The branches are as long as I feel like making them. I carry a bucket of water to traverse any lava that I come across.
If the drop is 23 blocks like in the diagram, they all just die on impact. I raised it up a block and now they're 2 hit kills. Seems like there's no good 1 hit kill range for some reason.
I think I know what your problem is. At the top of your drop you have water pushing the mobs into the fall. You need to change this so that there is a single dry block between the water and your drop. Mobs will crowd onto this block and be pushed off the edge. The reason you do this is that when you push the mob directly into the fall using water the mob will sometimes jump which makes the amount of fall damage taken inconsistent. If you do this and get the height right you will end up with skeletons that can be killed with a single punch.
Have you lit all the nearby caves and the surface of your world? There is a limit to the number of hostile mobs that appear at any one time. If there are too many hostile mobs outside your trap none will spawn inside. Ideally you would light up the entire surface and every cave within 150 blocks of your slime trap. While you may not be able to do this, at least light up the surface and clear the caves closest to your trap. The less places there are for hostile mobs to spawn outside your trap the more that will spawn inside.
I ended up modifying my 21x21 mob trap in a similar way except I added a water brake so that mobs always survive the fall (without the water brake damage was inconsistent depending on how many levels up they were when they fell).
They will walk off if the drop is 3 blocks or less I believe. If they walk off and fall into water the water will push them along and into a longer drop. However be warned that any mob trap the relies on the mobs moving to fall into water etc will not work if very well once you are more than 32 blocks away as mobs do not move around much when the player is not in range. Larger mob traps require either a piston or flushing water mechanisms to move the mobs around in the trap.
In the current release version of Minecraft (1.2.5) you can only find melon seeds by searching chests found in abandoned mineshafts. In the latest snapshots you can trade with villagers for melon slices which can then be used to obtain melon seeds.
I tried and failed to get mobs to die with 100% consistency from fall damage into water even using fences as others have suggested. I ended up creating a water brake so that no mobs died from the fall and then a lava blade to kill them and funnel the goodies to me. You can download my flushing mob trap test world if you want to take a look.
If you can plant the mushroom but it won't grow with bonemeal then you do not have enough space around the mushroom for it to grow. This could be because the ceiling is too low, or there may be a block within the 7x7x8 space required for the mushroom to grow.
Remember that the collection point can be up to 32 blocks below you. If you are building a piston funnel design that's 10 levels that it can be below the blaze spawner (3 blocks per level). You wouldn't necessarily stand above the spawner as shown in the video, a better spot would be on the same level or just below the mid-point between the spawners. I do like the conveyor idea but I'm wary of overly complex redstone designs.
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A description of your mob trap would be useful.
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No, as long as its not the environmental damage that kills them you receive full XP. Most XP farms take advantage of this by using environmental damage such as a fall or suffocation to reduce the mobs health to low levels to make killing them faster.
In terms of gaining XP 30 levels in 15 minutes is not too bad. If you are using a properly designed farm you can go AFK (Away From Keyboard) while the mobs build.
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I think I know what your problem is. At the top of your drop you have water pushing the mobs into the fall. You need to change this so that there is a single dry block between the water and your drop. Mobs will crowd onto this block and be pushed off the edge. The reason you do this is that when you push the mob directly into the fall using water the mob will sometimes jump which makes the amount of fall damage taken inconsistent. If you do this and get the height right you will end up with skeletons that can be killed with a single punch.
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http://www.minecraftwiki.net/wiki/Melon_Seeds
http://www.minecraftwiki.net/wiki/Abandoned_Mine_Shaft
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http://www9.zippyshare.com/v/72215801/file.html
This design nets me around 2,800 items/hour in my SSP world.
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