Rough idle black smoke when engines hot

Check plugs and leads, sounds like a chronic misfire problem, pull plugs, faulty cylinder will have a very black and sooty plug.

Do a wet test on the low cylinder, just a small squirt of oil to seal the rings, if the comp comes up it will be rings if not valves, do the test hot as well.

Normally not, but with 2 low comps we need to establish the cause of this, a problem with an inlet valve may impact on other cylinders. If you can do or get a leak down test done, this will isolate the main problem, such as which valve is leaking or if a ring problem.

You must find where the compression is going first, that type of test will be fairly inconsequential at this stage.

It's near midnight here in Australia, so I am signing off soon, let me know how you get on. If you need more help re post here.