Does it Affect Carburetor Setting?! Why Compression Pressure Measurement is Important to Know the Basic Strength of the Engine

One of the three most important factors for engine condition is good compression. This is because sufficient compression pressure in the combustion chamber increases the combustion pressure of the air mixture ignited by the spark plugs, resulting in ideal engine power. In the case of two or more cylinders, it is also important that the variation between cylinders is kept within a certain range. And a compression gauge is essential to know the compression pressure, which is important before setting the intake and ignition systems.

It is important to know and measure the absolute value of compression pressure, which varies from model to model

An example of a compression gauge for measuring compression pressure. While some gauges are designed to measure compression pressure by pressing a rubber plug on the end of a hose against a plug hole, this product is designed to measure compression pressure by connecting the gauge to a hose by screwing an adapter into the plug hole according to the plug's thread diameter. Since the gauge does not need to be pressed against the plug hole, there is no risk of the gauge coming off when measuring alone. However, note that the adapter may interfere with the plug-hole inner diameter on water-cooled engines and other engines with small inner diameters.

Compression pressure is measured with the throttle fully open. A fully closed or half-open throttle will not give the correct reading because there is not enough air to get into the cylinder. The question may arise as to what is the correct value, but the compression pressure set by the motorcycle manufacturer is the value measured with the throttle fully open, so we can compare and verify our measurements by opening the throttle fully. For kick-starting, the pedal is depressed vigorously, and for cell-motor starting, a fully charged battery is used to turn the cell.

The operating principle of the 4-stroke engine is that the intake and exhaust valves open and close to intake the air mixture and exhaust the exhaust gases, and when the intake and exhaust valves close and the piston reaches the upper dead center of compression, the pressure in the combustion chamber reaches its maximum.

Of the three major engine requirements - good air mixture, good compression, and good ignition - good compression is created here. In the case of a naturally aspirated engine, engine performance depends on how efficiently air is sucked in by the intake negative pressure generated as the piston moves toward the bottom dead center, and how well compression pressure is kept from escaping when the intake and exhaust valves close and compression increases.
If the intake and exhaust valves do not close completely for any reason, or if there is leakage of compression pressure due to worn piston rings or loss of tension, the combustion pressure of the air mixture will also decrease.

Whether a carbureted or fuel-injected vehicle, if the engine cannot generate the necessary compression pressure, it may not be able to achieve the proper mixture ratio. For example, an engine with reduced intake negative pressure due to worn piston rings will produce less air overall relative to the jet size and ECU programming. If this is interpreted as a misalignment of the carburetor settings, subsequent work can also go in the wrong direction. To prevent this, it is important to know the compression pressure of the engine.

The gauge used to measure compression pressure is a compression gauge. An adapter that matches the plug thread diameter is attached to the cylinder head, and a hose is connected to the gauge body to turn the crankshaft. The main rule is to open the throttle to full throttle when measuring, whether kick-starting or starting a cell motor. For the cell motor to start, press the cell button for a few seconds and read the value when the gauge needle reaches its highest limit. Note that in the case of two or more cylinders, if plugs other than those in the cylinder to be measured are left in place, compression pressure loss will occur, so all spark plugs must be removed before the measurement is taken.

As internal engine wear progresses, pressure generally changes in the direction of decrease, but in rare cases, such as when carbon sludge is deposited on the piston tops due to oil buildup from a carburetor setup error or worn piston ring oil rings, the combustion chamber volume may decrease and compression pressure may increase. This is a rare case where the combustion chamber volume is reduced and the compression pressure increases. Most importantly, always refer to the service manual for each model to determine if the compression pressure indicated by the compression gauge is normal or not.

Sometimes we hear people say that the compression must be kPa, but the figures vary greatly from model to model.
As far as I could confirm from the service manuals I have at hand, the Honda Monkey of the 6V era had 1000 to 1200 kPa, and the Jol Cub of the same horizontal type engine had 1373 kPa. The Yamaha SR400 is 850-1050 kPa, while the Naiken, a liquid-cooled three-cylinder engine, has an incredibly high compression pressure of 1331-1713 kPa for an out-of-print model. Also, the standard value for the Kawasaki Z2 is 1050 kPa, while the Zephyr χ has a tolerance range of 775 to 1180 kPa. The GSF1200 with an oil-cooled engine was 860-1230 kPa.

The fact that there is a large difference in the proper compression pressure between these two models should tell you that when measuring the compression pressure of your car, it is important to use the standard value (pressure listed in the service manual, etc.).

The difference in compression pressure between cylinders is analogous to a malfunction in the engine itself.

For 4-cylinder engines, remove all spark plugs before installing a compression gauge to take measurements. When cranking is started by pressing the cell button, the gauge's pointer gradually rises as it reaches the upper dead center of compression, and the value is read at the point where it stops rising. If there is a large pressure difference between cylinders, add a small amount of engine oil through the plug hole and measure again. If the compression pressure rises at this time, check the engine body, specifically the piston rings and cylinders, to see if wear has progressed.

While the specific value of good compression varies from model to model, it is also necessary to pay attention to the difference in compression pressure between cylinders in a multi-cylinder engine. As mentioned earlier, compression pressure is within the proper value when all intake and exhaust valves are closed and the blowout from the piston rings is within a certain range.

In a multi-cylinder engine, valve closure and piston ring tension are generally the same when the engine is new, but subsequent wear and pressure drops do not always progress evenly. For example, if carbon sludge gets into the exhaust valves and makes them less airtight, compression pressure will drop in that cylinder, and the resulting deviation can affect combustion conditions and even carburetor settings, causing the overall condition of the engine to deteriorate.

Some models indicate the difference in compression pressure between cylinders as a condition for determining such a problem. In the previous example, the Suzuki GSF1200 has a 200 kPa difference between cylinders, and if the pressure difference is greater than this, it is stated that there is some problem on the engine side and that inspection is required.

The standard value for the GSF1200 is 1230 kPa, so if all three cylinders are around 1200 kPa and only one cylinder shows 1000 kPa, it means that the engine needs to be adjusted. In general, if the compression pressure is around 1000 kPa, or about 10 kg/cm² in the old unit system, the engine should be fine, but in the case of a multi-engine, the difference between cylinders is also an important criterion.

In the case of carburetor cars, the spark plugs tend to burn thinner as the intake negative pressure = the cylinder's sucking power decreases and the compression pressure also decreases, preventing sufficient air mixture from being sucked in. Judging this only by the burnt condition of the plugs and making the jets thicker will not improve the symptoms because the compression pressure is insufficient in the first place. It is true that plug burnout is an important criterion for carburetor setup, but good compression is a prerequisite for this. On top of that, it is important to know that in the case of multi-engine, it is important to understand not only the absolute value but also the relative difference between cylinders.

 The condition of the engine itself is often neglected in the modification and tuning of the intake and ignition systems because the engine can run without excesses or deficiencies. Although it is not easy to disassemble and inspect the engine itself, it is recommended to measure the compression pressure at least once, since the internal condition of the engine can be determined simply by attaching a compression gauge to the plug hole and pressing the cell button.