Techniques for Removing Carbon Deposits in the Combustion Chamber

Gasoline, which is primarily composed of carbon and hydrogen, breaks down into carbon dioxide and water when completely burned, but when used as a fuel in internal combustion engines, it does not always burn ideally. However, when gasoline is used as fuel in an internal combustion engine, it does not always burn ideally. As a result, combustion residue inevitably remains in the combustion chamber. Also, some of the engine oil can get into the combustion chamber and cause it to burn. These carbon deposits can damage the performance of the engine, so it is important to remove them as soon as possible.

Carbon deposits are the residue of burning fuel and engine oil

This is the combustion chamber of a Honda Monkey that has only travel about 2000km since it was new. When I removed the cylinder head to increase the bore, I also removed the valves, but the carbon deposit was as soft as cocoa powder and was so light that it could be easily removed with a brass brush. The carbon deposit was so soft as cocoa powder that it could be easily removed with a brass brush. This level of carbon deposit could be cleaned with a chemical injected into the fuel tank.

This is the combustion chamber of a Kawasaki KZ900 with only about 6,000km on the odometer, but it's been over 40 years since it was manufactured. The carbon deposits on the combustion chamber peeled off like scabs. At first glance, it may look easy to remove, but if the carbon deposits get caught between the valve and valve seat, the valve will not seal properly.

The black, hard product on the piston tops, combustion chambers, and around the valves is called carbon deposits. Carbon refers to the carbon contained in gasoline and engine oil, and deposit means to accumulate or deposit sand or mud.  Apart from this, it can also refer to money deposited in a bank or other institution, a deposit, or a security deposit. In other words, it is an accumulation of carbon.

When hydrocarbons such as gasoline are burned, even in an alcohol lamp, the chemical reaction equation is carbon dioxide and water. However, this is only a theory, and the ratio of the two is constantly changing in the actual engine, with excess gasoline or excess air. The theoretical air-fuel ratio is 14.7 to 1 (14.7 grams of air to 1 gram of gasoline by weight), but when the air-fuel ratio is measured on a chassis dynamometer, the value varies greatly depending on the engine speed. Even if the theoretical amount of gasoline is fed into the combustion chamber, it does not mean that all of it will be burned completely. Excess fuel is not burned off, but is deposited as carbon on the piston tops and in the combustion chamber.

It is not only the mixture of gasoline and air that enters the combustion chamber. What else? Blow-by gas and engine oil also enter the combustion chamber. The crankcase is equipped with a breather pipe to relieve the pressure inside the case, and this pipe is connected to the air cleaner box. When the mixture burns and generates a lot of pressure, most of it is used to push the piston down, but some of it slips through the gap between the piston, piston rings, and cylinder and flows into the crankcase. This is the blow-by gas.

Blow-by gas, which contains unburned gasoline, is harmful to the environment, so nowadays a breather pipe is required to reduce the gas from the air cleaner box back into the combustion chamber. The mist separator in the crankcase is a part that prevents this from happening, but blow-by gas is always flowing, and the path of the breather pipe is limited in length in a motorcycle engine that must be compact, so it is difficult to completely separate the oil mist.

By the way, you may have seen a clear tube with a cap inserted into the air cleaner box and emulsified oil collected at the end of it, but that is part of the oil mist that has collected in the box through the breather pipe.

When this oil mist adheres to the intake valves and combustion chambers, it burns and deposits at high temperatures. In other words, both gasoline and engine oil mist are the cause of carbon deposits.

Carbon remover for initial buildup, a head overhaul for severe cases is the best bet.

Paint stripper has the ability to dissolve carbon deposits. In order to increase the retention rate, a high viscosity paint remover is used and spread over the combustion chamber and intake and exhaust ports with a toothbrush. The exhaust port is where the high-temperature exhaust gas from the combustion chamber passes through, so it is difficult for deposits to form, while the intake port, where the temperature is lower and the oil mist from the blow-by gas does not stick, will have more deposits.

Just scrubbing it with a toothbrush cleaned it up completely like this. When using a metal brush, be careful not to scratch the surface of the valve seat where the valve contacts. If it is as hard as a brass brush, it is fine.

Even with the most precise fuel injection and piston clearance control, carbon deposits will never be reduced to zero as long as there is still fuel leftover and blow-by gas is generated. Excessive buildup of carbon deposits will reduce the combustion chamber volume, and the deposits themselves will store combustion heat, causing heat spots and abnormal combustion such as self-ignition and knocking.

In order to prevent such problems, there are chemicals called carbon removers. There is the liquid type to be mixed with gasoline, and spray type to be sucked in by installing bypass pipe to the intake system, but all of them appeal that the cleaning ingredients adhere to the carbon in the valve and combustion chamber, make it float up, and burn it in the combustion chamber and exhaust it.

I don't know what any product is capable of as I haven't tried them all. Each product must have been thoroughly tested during its development, so it should definitely be effective. However, the carbon deposit that forms in the combustion chamber varies from one to another. Naturally, the situation will be different between an engine that has been riding for 1 year and 8,000km from new and an engine that has been riding for 10 years and 60,000km. Even if the type of motorcycle and the mileage are the same, it is said that the way of carbon deposition changes depending on the way of driving.

If you start riding a new motorcycle and use carbon remover regularly in the span recommended by the product, you may be able to control the formation of carbon deposits. On the other hand, if the motorcycle is used and has a good amount of mileage on it, it is hard to tell how effective the carbon remover is. Recently, inexpensive fiberscopes that use a smartphone as a monitor have become popular, so you can check the piston head through the plughole.

If you are still worried, the best thing to do is to remove the cylinder head and remove the carbon directly from the combustion chamber and valve area. If you remove the head, depending on the model, you may need to lower the engine from the frame, you may need gaskets, you may need to take care of the coolant if it's a water-cooled engine, and above all, you may need a lot of work and some experience. The hurdle is definitely higher than just injecting carbon remover into the gas tank and being done with it. However, if you want to remove the carbon for sure, there is no better way than this.

How to remove stubborn carbon? Is it by chemical or baking soda blasting?

The combustion chamber of the Yamaha Tricker, which has been riding for 40,000km since new, was heavily covered with carbon deposits, although it did not show even the slightest hint of malfunction in normal driving. With the stock carburetor and OEM exhaust system combination, the carburetor setting seemed rather thin, but I wondered if this was caused by too much blow-by gas. The deposit is like baked on, so the type of chemical that is injected into the gas tank may not be effective in this situation.

Spray a generous amount of foam-type carbon cleaner. Soft deposits will dissolve immediately after spraying, but for stubborn deposits, it is best to leave the cleaners for 1 to 6 hours to allow it to penetrate. Some carbon cleaners as well as strippers can damage the paint, so wipe it off as soon as possible if it gets on the painted surface. It is also advisable to protect the painted surface of the engine with masking.

There are still some deposits around the plugs but most of them were removed. The valve seats were also cut at the internal combustion factory to remove the carbon deposits. The performance of the engine is sure to improve after this refresh.

Once you've made the decision to do carbon removal on the cylinder head alone, you'll need to figure out what to use. The simplest method is to scrub with a brass brush and kerosene. If it is a light carbon, this will be enough to clean it. Conversely, if it is that much carbon, you could have used a gas tank injection-type chemical.

The next most common and recommended method is to use a chemical that has been developed as a carbon cleaner. The foam type spray type is popular, and even stubborn carbon deposits can be removed by spraying and waiting a few hours to soften the tickly carbon. The gasoline additive type burns with combustion, and if the cleaning ingredients are strengthened and the combustion becomes bad, the engine will stop.

Carbon removal using paint remover has been handed down from generation to generation by veterans who have been tinkering with motorcycles long before carbon cleaners became popular. It is the same as the foam-type spray chemical, just apply it to the carbon deposit and wait for a while. There are two types of paint remover, a low viscosity type, and a sticky gel type, but we recommend using the gel type to ensure sufficient reaction time.

Finally, I will introduce baking soda blasting as a method that is extremely effective in removing carbon deposits from the combustion chamber. When we think of blasting, we tend to think of sandblasting using alumina or glass beads, but EZ Blast (Easy Blast), a typical baking soda blasting method, uses sodium bicarbonate (baking soda), which is also used as a baking powder, as an abrasive.

Baking soda is a solid abrasive that is injected with pressurized air from an air compressor, but it does not have the same grinding power (ability to scrape metal and other materials) as alumina or glass beads. However, when struck at high speed, it powerfully pulls carbon out of the combustion chamber and intake ports. In addition, EZ Blast can be used as a wet blast with tap water, allowing you to spray baking soda and wash it off at the same time. Sandblasting with alumina or glass beads requires a lot of work to remove the abrasive afterward, but since baking soda is water-soluble, rinsing with water or hot water will not leave any solid residue that could affect the valve stem or piston. If there is any residue, there is no need to worry as it is not hard enough to damage engine parts.

Using baking soda blasting is expensive to install because it requires a special machine and a compressor with horsepower, and it's not as easy as spraying a foam-type chemical in your garage or yard, so not everyone can do it easily, so it's good to know for reference.

Whichever method you use, the efficiency of carbon deposit removal will be greatly improved by using a single cylinder head. If your mileage has increased, if you have bought an old motorcycle second-hand, or if you are concerned about knocking under acceleration, you should first try the easy tank injection type carbon remover. If you still can't see any clear results and the carbon deposit accumulation is not reduced by looking through the plug holes, you may want to try to remove the carbon deposit from the cylinder head itself.

The carbon deposits on the Kawasaki KZ900 are less severe than those on the Tricker, but they are perfectly fixed after a long period of aging. Because of the low mileage of this old machine, the inner circumference of the valve seat (front side) of the intake port shows a metallic sheen.

The EZ Blast system adds pressure from the air compressor to the baking soda in the light blue tank at the rear, and then combines it with tap water at the nozzle at hand. Sandblasting with glass beads or alumina in an open-air environment like this would be a disaster due to the abrasive material that is scattered all over the place, but baking soda is water-soluble, so it dissolves in water and flows away after the carbon deposits in the combustion chamber are removed. Also, since there is no residue on the metal or glass, the engine can be reassembled after it is flushed with water and dried.

No scrubbing with a brush, so you can literally remove carbon deposits without scratching. EZ Blast also has the added benefit of cleaning the outside of the engine as well as the combustion chamber. It can also clean the carburetor and remove rust from the chromium plating.

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