cleaning up the slant six head

Lets begin with the FACT vinegar is commercially produced in a "generator" heated retort type vessel made of cast iron. Vinegar is a nasty product in that it is an acid, and not particularly friendly to iron. Vinegar generators have a life expectancy of about 6 months, and all plumbing to and from them is PVC, and holding tanks are usually poly. Vinegar will also destroy copper, although it lasts longer.

Vinegar will dissolve rust, BUT, vinegar does NOT stop attacking iron molecules when it reaches good metal. Vinegar is a poor rust remover given that it damages iron.

WHY does freshly derusted iron flash rust so fast, and what can be done to stop that?
Well gee, you just exposed pure iron containing molecules to oxygen and provided the catalyst that makes rust react. DOH!
You want to stop rust from happening, remove the catalyst instantly by heating the casting or iron object to 300° in a water free atmosphere, and that ain't your gas grill that produces water from the burning propane.
Motor oil and transmission fluid don't cut it either because both are formulated to absorb water.
WD 40 ain't getting the job done either, it's mostly Kerosene and designed to DISPLACE water, not stop water.
Got a lot of money to waste, look at Boeshield or Fluid film. Both are mixes of lanolin and mineral oil with a carrier. You can make it for less than 1/4 of what you can buy it from either manufacturer.

On the subject of carbon. Carbon is an element, it cannot be broken down in the manner rust can. Rust is 2 elements, and can be broken apart by removing either the iron atom or the oxygen atom. Both Iron and oxygen are elements and cannot be broken down although both can be combined with many other elements fairly easily at room temperature. Carbon does not like to combine unless a lot of temperature is added to the equation.

Yes, there used to be commercial hot tanks used in engine shops. They were steel tanks with a high capacity burner and most had agitators, and they were usually ticking time bombs. NO, they have not been outlawed by EPA or anyone else. EPA and the Possum Police in every State have placed books of rules on the operation of these tanks with cradle to grave registration of the chemistry the tank operates on. That costs far more money than the tank can generate in any production situation today when added to the cost of fuel and INSURANCE.

Are there effective room temperature carbon removers? Yes. Look at a can of EZ Off or any oven cleaner.
These work not by dissolving carbon, but rather than breaking down the material adhesing the carbon to the metal, and require time and a friendly environment to function. THEY CAN ALSO CAUSE SERIOUS SKIN BURNS AND BLIND YOU IF YOU GET SOME IN YOUR EYE.

Since carbon cannot atomically combine with iron below the melt point of the iron, we know the carbon is stuck to the engine by a adhesive bond. This is different from "Mill scale" on new steel from a mill. We also know most castings are wonderful adhesion surfaces because they are rough. Removing carbon from an engine increases in difficulty tremendously because the carbon has been impinged onto the surface at high temperature, and grabbed a foothold on both the roughness and porosity of the metal. Then it was baked into place.

Since carbon has no propensity to go into solution with anything else at room temperature, it ain't coming away from iron easily. There seems to be a theory in the cooking industry that carbon comes off easily with a cleaning agent. Most of the cleaning agents offered for sale at high price are strictly dissolvers of the grease holding the carbon adhesed to the iron, employing the same group of ingredients found in paint strippers. These are some nasty products that will eat your skin and blind you. If you insist on using them, have plenty of BEER standing by. Beer is the best neutralizer that will stop the burning,actually the consumption of fat cells in your skin by the corrosive ingredient. If you don't need the beer for first aid, you can always drink it as you admire your work.

Want an easy & fast wat to remove carbon and carbon contained in fully plasticized grease from metal? Liquid Nitrogen. It breaks the bond fast. It will also cost you a fortune for the Nitrogen and equipment to handle it. It's damn fast though.