I'm in full agreement with not using anything that can accidentally bend your connecting rods. During the tear down phase of an engine head R&R... I was really shocked at how much carbon had steadily accumulated inside my son's 2002 TB engine. (Before and After Cleaning Images follow and many more are available at the link below...)
During the subsequent efforts at cleaning up this mess.. I discovered from
@Mooseman that the only effective method for dissolving this dreadful material from within the combustion chambers and on the tops of the pistons and top rings... was a heavy application of GM Top Engine Cleaner. The stuff is quite foamy and remarkable in how it dissolves all the Carbon like nothing else can.
At the time I was using this on the 'wide open engine" it occurred to me that since this engine's spark-plug holes open up into the upper most area of the combustion chambers.... that on a fully assembled engine...it would indeed be laborious... but after removing all of the spark plugs... an intrepid mechanic could use a long spray nozzle and after bringing each piston to Top Dead Centre ...one after another following the correct Firing Order of the GM Atlas Engine... it would be possible to fill up the cylinders with the spray foam and leave the engine sit for 10-15 minutes...one after the other...again with no spark plugs installed...and after stuffing paper towels down into the spark plug wells to absorb the carbon-soaked excess fluid in each cylinder, rotating the engine in a clockwise direction very gradually to allow the liquid to be absorbed or pushed out and around the exhaust valves.
You could also use compressed air attached to a compression tester hose to slowly pressurise each cylinder and evacuate the foamy Carbon laden fluid before re-installing all the plugs and hand-turning over the engine very slowly a few times to ensure no hydro-lock of the cleaning fluids would occur. It follows that because of the serious penetrating power of this stuff... it would be wise to perform a Mobil1 Oil and Filter Change as the contents of the crankcase will have been contaminated during this cleaning effort. It is important to mention that NO SCRUBBING WHATSOEVER WAS NECESSARY TO ACHIEVE SUCH CLEANLINESS!
I will agree that most of the time... this kind of complicated combustion chamber relief will not be practical and most people will have neither the experience nor the confidence in doing something that seems so drastic. But FWIW...look at the Before and After images of the Pistons and Cylinders on an engine with 240,000 miles and you'll see why doing this over a long weekend when your certain not to need the vehicle... with patience and care...it will get rid of an enormous amount of Carbon build-up that only serves to cause pre-ignition...changes the "quench" dimensions inside the CCs ... and robs the engine of power and smoothness:
http://s557.photobucket.com/user/60...NEREPAIR/TRAILBLAZEREPAIRTOOLS?sort=3&page=11