I was going to let others maybe comment before I did on your last post.......as I said in the past and was going to say again, but you answered your own question......"carbon, Carbon, CARBON".
I'm sure that's what's going on. From what I've read about that motor is that the valve springs are weak and can be pushed in by hand. That is not a lot of pressure on the "Valve Seat" which I believe you are talking about and didn't know the name of. Minimal pressure would allow build up of carbon since the valve does not have a lot high pressure tension on it (valve seat pressure) which would keep the valve clean. That's what I think has happened on your motor.....in addition, oil is probably getting past the rings because they ring pack is seized up with carbon allowing more oil to be burned creating more combustion chamber carbon. I'm sure the valve seals are also probably allowing oil into the upper valves causing again, more carbon, but that could be controlled if everything was cleaned with continuous treatment of the fuel or the seals can be easily replaced without removing the heads........
I don't know if it's possible to bend valves on that motor.
I think somewhere on this thread I gave you some idea's of what to use to dissolve carbon other than water.
The old Berryman B12 Chemtool #0116 product we have here in the U.S. since the 1940's is a very strong solevant.
I found my notes from their chemists who told me it can be re-produced if you can not find it:
A ratio of::::::: 2 parts Tolulene : 1 part Acetone : 1 part MEK ( Methyl ethyl ketone) 2 liter of Tol, 1 liter of Acetone, and 1 Liter of MEK. Maybe mix in some light weight oil for lubrication and to extend the "wet time" since they will evaporate very quickly....... These chemicals can be purchased at your local paint store.
The trick would be to somehow soak the backs of the valves. You really have to put some thought into this for an aggressive treatment and soak which will also help dissolve carbon in the ring pack.
Here's maybe one way to do it. Remove all spark plugs, remove the coil wire, place towels over each bank of spark plug holes, crank the motor for about 5 seconds while pouring in a 1/2 liter of this mixture down the carburetor. Maybe use a weak battery to crank slowly. Allow to soak for 24 hrs. Hopefully some valves will end up closed and keep the mixture in contact with the seat/valves, but it will probably all leak past within a few minutes to seconds. Do the procedure again for maybe 3 seconds, and hopefully other valves will end up closed allowing the liquid to soak the valves/seats. Do this a few times over the next few days, and drain the oil or take the drain plug out while doing this. Keeping the plugs out will ensure you don't get hydro-lock, but I'm sure all of the fluid would drain into the pan within minutes or within a few hours. If it doesn't, you will know which ring packs are clean and tight because when cranking, all of the fluid would shoot out of the plugs holes.
Maybe even loosen all of the intake rocker arms, and just pour the mixture down the carburetor and fill up the intake manifold and allow it to soak into the combustion chamber, but keep the plugs in this way you flood each combustion chamber and fill it up so the level stays above the valves. It will eventually soak past each piston and out the oil pan. I would collect the mixture, and filter it through rags, and reuse it and keep pouring it down the carburetor. You may even need to drill a small hole into the lowest point of the exhaust pipe under the exhaust manifolds to drain any fluid that may get into the exhaust or remove a lower section of exhaust to blow out the mixture when cranking......
Allow it to soak for a few days to soften the carbon. Then do a rinse with kerosene / diesel fuel mixed with light weight oil to re-coat all parts with some oil before start up and to flush out the Tolulene/MEK/Acetone mixture. After you flushed it out, maybe leave the plugs out, and drain plug to allow all of the Tol/MEK/ACT to flash-off and evaporate for 24 hours. Try to spin/rotate the valves by hand back and forth. Tighten all of the rocker arms to specification, put a few squirts of oil into each cylinder, start/crank the motor without the plugs, install plugs, add oil, and start it up and allow to idle up to temperature, then use water or carburetor cleaner down the carb while revving the motor, then take it out on the road and do full speed passes/runs to blow out carbon.
That's an extreme approach. I and some of our old fleet mechanics have done similar procedures and our Delivery Trucks with Big Block Chevy Motors. They survived and no reason the Caddy motor couldn't.
Just think about what your doing and take a methodical approach.
You could always just keep trying the fuel additives, or just remove the heads to clean the valves manually, and then do a piston soak when the heads are off and allow some strong solevant to sit on top of the pistons.......
One of the best ways to dissolve carbon is ammonia based paint stripper or even 10% ammonia by itself but that would wash down the cylinder walls. There is even restaurant Grill and Oven Stove Cleaner, which is Tolulene or Amonia based that dissolves hard carbon on stoves, grills, and cooking pots.
I think I mentioned somewhere here about some strong but factory approved carbon cleaners:
GM Piston and Ring Cleaner #12378549. It should be available in Europe, but is expensive but will be hard to direct it to clean the valve with the heads on. Also GM CLEENS also known as GM Top End Cleaner (TEC) but is no longer sold in the U.S. but can be found on e-bay:
http://www.j-body.com/forum/showthread.php?21945-GM-Cleans-meets-a-high-mileage-motor-56K-com-back-tomorow
Or; MOPAR CCC (Combustion Chamber Cleaner)
Here is a specific TSB and specific technique on using it:
http://jameskbeard.com/jameskbeard/Cadillac_Files/02-06-01-009C_Oil_Consumption.pdf
There was also a cleaner I posted about that is popular to dissolve firearm / gun carbon called "Ed's RED Bore Cleaner".
Here is a very well known product that is environmentally friendly, but is used to soak parts after dissembled. I've talked to the mfg. about using in the engine, but they said their concern would be flash rust on iron like the cylinder walls.
http://www.orisonllc.com/pistonkleen.html
Depends on how aggressive you want to get with treating this condition, but this motor appears to be badly carboned from my initial thoughts before you did the leak-down or compression test which seems to indicate it. After this aggressive carbon soak, I'm sure the compression test and leak-down will improve significantly.
Once again, the engine can not be efficient for fuel economy if it has ton's of carbon.
Maybe others can add their concerns, thoughts and ideas...
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By the way, to answer your question about what is acceptable for leakdown specs....
Here's another article on the subject and they confirm anything more than 20% for any engine is too much.
I still believe you have carboned valve seats and ring packs:
http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/viewall.html
Simple tips:
http://www.cadillacforums.com/forums/northstar-engines-system-technical-discussion/95294-cylinder-leak-down-test-procedure-percentage.html
Old TAVIA Tool Instructions:
http://web.archive.org/web/20040225191535/http://www.tavia.com/08015_instructions.html