Re: 1 cylinder, high compression, the others are equal
I hadn't read TR's cure, he says to do it at higher RPM under load and cautions against doing it too much. That's much safer that what most people do.
Most folks run it at high idle and pour it in a little at a time from a cup.
I had one of my mechanics do that as a "favor" for a customer. A chunk of carbon dislodged and then lodged on the top of the piston - creating a terrible knock. I made him rent a car for the lady while he pulled hers down to fix it. Fortunatly it didn't do any damage, slightly larger it could have really made a mess. It was just banging against the cyl roof.
Most of the time I agree with TR's diagnosises and cures, obviously he's had lots of experience. I have too, but from a slightly different aspect. This is one time I differ a little.
I just don't like adding water.
The idea behind it is twofold - one it does the same thing as steam cleaning, cleaning all the gum and dirt out. The rings actually don't touch the walls, they ride on a slight film of oil. Take away that oil - well you can imagine!
Also the cold water hitting the glowing red hot carbon chunks shatters the carbon, breaking it off the chamber. It gets it out, but if there's big pieces one may get stuck under a valve, when the piston comes up ----.
Or, like what-his-name's did, lodged on the top of the piston and hit the roof.
It's best to add something that will dissolve and remove it slowly. But that's only a temporary fix if you don't cure the original reason it's carboning up in the first place.
Reasons: Anything that causes incomplete combustion will create excessive carbon. Carbon is just the result of incomplete burning, usually a lack of oxygen for the amount of fuel. If there was enough oxygen, it would turn the carbon into CO and CO2 gasses.
Rich mixture, cold plugs, timing way off etc, possibly even a partially plugged exhaust.
Plugs I've already addressed, timing either fast or slow can cause improper burning, or improper advance.
Usually it traces back to too much fuel for the air = Rich mixture. Either at idle or higher speeds or both.
Now I'll throw a curve - can also be a vacuum leak. (Vac leak? That's lean! HUH? IS THIS GUY NUTS?)
Let's take a simple straight line 6 cylinder with a log type intake manifold. Let's introduce a vacuum leak at the rear - a loose line or even a gasket. Air gets sucked in through that leak, affecting the rear cylinder - it's lean, not enough fuel.
The engine runs rough.
Abdul says "Me Fix" and adjusts the carb richer, till the engine runs smooth again. "Fixed, $30 please!"
But what really happened? Now the rear cylinder is getting enough fuel so it no longer misfires - good. But, the others are now too rich. After a while those build up lots of carbon - all except the one that's running right. (Hmmmm, sound familiar?)
Computer controlled cars are famous for that. Depending on which side the leak is affecting, and which side the O2 sensor is on, it could run either rich or lean. If the leak affects the same side the O2 sensor's monitoring, then the sensor sees the excess oxygen and richens all cylinders - you never know it till later or by checking your mileage records. If it's on the other side, it just runs rough.
The best method to find a vacuum leak is use propane, a controllable fuel that burns like gasoline.
Take a propane torch's orfice and head off. Slip a vacuum hose over the end about 4 feet long.
Holding it upright you get vapor propane, upside down you get liquid for more volume.
Let a little vapor go down the carb or intake - with or without air cleaner on. Note the engine's reaction.
It normally should "like" the propane slightly, RPM should increase just a tad, but no more than about 100 RPM. If it "likes" it too much, it's too lean. Same applies for high speeds, and even under load. I use it all the time with a chassis dyno and an engine dyno. It's a real easy way to see how a richer condition helps or hinders.
If it slows down it was too rich - you are making it even richer.
Vacuum leaks - simply spray the propane around places you suspect a leak - if the RPM changes you've hit it. Remember the propane blows around, so don't be fooled by it blowing in the intake when you spray a heater hose. Keep the fumes away from the distributor - I think you can guess why.
If you have trouble locating a leak, you can lean the carb down till it starts to run rough, then when you hit that small leak it will be really obvious.
Computer controlled cars are a little harder, when the propane hits the leak there will be a slight RPM change, then it will drop back to normal as you hold the propane going. The computer sees the rich mix and leans the system out as well as readjust the idle speed. Then when you stop applying propane, the RPM drops for an instant as it has to compensate again. You just have to listen more carefully.
I like propane rather than a liquid, like a burnable carb cleaner, ether or whatever. Liquids puddle, then if they ever catch fire you have a problem, propane just flashes, scares hell out of you, then goes out.
Also a liquid can wash dirt into the hole temporarily plugging it. "It was there a moment ago!"
Base gaskets, hoses, power brake boosters, trans vac modulators etc can all be letting excess air in.
I know this is much more detailed than you asked, but hopefully this helps a few other folks as well.
Almost always the carbon will gently and safely burn off as you drive it after the cause of the problem is cured. The addatives simply help it a little faster.
Let us know what you find.