When an engine experiences a performance failure, it needs to be repaired to restore its good technical condition. At this point, the engine needs to be disassembled, cleaned, and inspected. For motorcycles, although the models are different and the repair methods vary, they generally include three major tasks: disassembly, cleaning, and assembly. Mastering the key points of these three tasks will greatly assist in motorcycle repair work.

The inspection before engine disassembly has two meanings for an engine that needs repair. First, for engines that are not faulty but require disassembly and assembly during the repair process, the inspection can help understand the original assembly situation of the engine, facilitating reassembly, which is particularly important for maintenance personnel without a motorcycle repair manual. Second, through the inspection, combined with the motorcycle user's description of the engine's failure, the location and severity of the fault can be more accurately analyzed and determined, so as to decide on the repair method for restoration.

The disassembly of the engine can be divided into partial disassembly and complete disassembly. This mainly depends on the location of the engine failure and its severity.

The principles and precautions for engine disassembly and reassembly

A. Principles of engine disassembly. Engine disassembly is generally carried out in a sequence from simple to complex. During the disassembly process, it should follow the principle of disassembling the external parts of the engine first, the internal parts second, the accessories first, and the components and parts last. The reassembly principle of the engine is generally carried out in the reverse order of disassembly.

B. Preparations for disassembly:

1. Engine disassembly must use special tools and standard tools. It is strictly forbidden to disassemble rudely to avoid damaging the engine components. Engine disassembly must be carried out when the engine body is completely cooled. Otherwise, it is easy to cause deformation of certain components and reduce the quality of engine reassembly.

2. To avoid deformation of components during engine disassembly, especially for main components such as the cylinder head and crankcase, the tightening screws or nuts should be loosened and tightened symmetrically and evenly. It is strictly prohibited to loosen or tighten one side of the screws or nuts before loosening or tightening the other side.

3. Mark the following components or components during disassembly: valves, pistons, the meshing position of the engine's intake and exhaust timing gears, all connection wires' connectors, etc. Other components can be reassembled according to the original motorcycle repair manual.

4. After disassembling the engine components, they must be classified and placed using parts car or parts disk. To avoid mechanical wear such as磕碰划伤 of the components； the disassembled screws or nuts should be reinstalled in their original positions as much as possible. To avoid damage or loss.

C. Cleaning of components. This is because the disassembled components of the engine have grease and oil stains on their surfaces. They must be cleaned thoroughly to ensure the smooth progress of inspection， classification， and repair work of the components.

The quality of the cleaning work directly affects the inspection and repair quality of the engine components. It must be done carefully， and at the same time， attention should be paid to saving cleaning oil and preventing waste. The cleaning methods are as follows. To make it convenient for disassembled components to be cleaned， different cleaning methods of components， such as steel parts， aluminum parts， and non-metallic parts， should be placed separately.

Components are generally cleaned with fuel (gasoline， kerosene， diesel)， and in conditions with repair facilities， they can be cleaned with alkaline solutions or ultrasonic cleaning machines. This cleaning method is relatively economical. The currently widely used method is the fuel cleaning method. The fuel cleaning equipment is simple and is suitable for maintenance and servicing in small repair sites. However, special attention must be paid to the safety of the maintenance operation to prevent the occurrence of fires and other disasters.

The principle of cleaning is to first clean the internal components of the engine, then the external components; first clean the main components of the engine, then the secondary components; first clean the relatively clean components of the engine, then the relatively dirty components; first clean the small-sized components of the engine, then the large-sized components; first clean the smooth components of the engine, then the rough components. After cleaning, the engine components should be placed back in their original positions according to the system to avoid confusion and mistakes.

After the engine components are cleaned, they should be dried with compressed air or wiped dry with a clean cloth. For the crankcase and individual components with lubrication channels or holes, compressed air should be used to clear the lubrication channels. The joint surfaces of the components must be cleaned thoroughly. Note that non-metallic components (such as electrical components) cannot be cleaned with fuel cleaning. Non-metallic components (such as friction plates) cannot be cleaned with kerosene or diesel, and gasoline can be used when necessary. Aluminum components are prohibited from being cleaned with alkaline water to prevent the components from being corroded.

Remove carbon deposits. During engine operation, due to incomplete combustion of the combustible mixture and other reasons, carbon deposits will form in the engine combustion chamber, valve top, piston top, etc., which will affect the heat dissipation of the components, reduce the engine power, increase fuel consumption, and shorten the engine's service life. Therefore, carbon deposits must be removed after the engine has been used for a certain period of time. In maintenance operations, fuel, bamboo tools, wooden tools or copper wire brushes can be used for removal, and iron tools should not be used to scrape off the carbon deposits, as iron tools are likely to cause damage to the components and shorten their service life.

D. Engine maintenance should follow the technical requirements and standards specified in the motorcycle maintenance manual provided by the motorcycle manufacturer. The motorcycle maintenance manual is an explanation of the operation essentials for motorcycle maintenance. In engine maintenance operations, the motorcycle maintenance manual should be used as much as possible to avoid mistakes caused by incorrect operation. Taking the disassembly and assembly of the camshaft of the Guangyang Haomai 125 type sitting motorcycle as an example, the disassembly procedure and requirements are introduced: (Due to the limited space, only a small part of the motorcycle maintenance manual is excerpted and the specific terms in the article are processed into general words.)

Disassembly of the camshaft:

1. Remove the four fastening screws of the cylinder head cover at an angle for 2-3 times, and remove the cylinder head cover.

2. Rotate the magnetic motor rotor in the working rotation direction. During the engine compression stroke, align the "T" mark on the top of the magnetic motor rotor with the timing mark on the engine body. The timing transmission chain wheel on the camshaft should be in the upward position and perpendicular to the cylinder head joint surface (the camshaft cam is downward in an inverted eight shape). This is the compression top dead center position of the engine.

3. Remove the adjusting cover screws of the timing transmission chain, rotate the timing transmission chain adjusting device clockwise to lock it, and loosen the timing transmission chain.

4. Remove the fastening nuts of the camshaft rocker arm bracket at an angle for 2-3 times, remove the fastening nuts, washers, and the camshaft rocker arm bracket and fixed pin.

5. Remove the timing transmission chain from the camshaft timing transmission chain wheel, and remove the camshaft assembly.

Component inspection:

1. Camshaft: Check if the working surfaces of the intake and exhaust valves and the cam height are damaged. Check if the camshaft bearings are loose or worn. If so, replace the entire set.

2. Camshaft rocker arm bracket: Use a 5mm bolt to remove the valve rocker shaft, take out the one-way check valve, valve rocker. Check if the camshaft rocker arm bracket, valve rocker, and valve rocker shaft are worn or damaged. When the sliding working surface of the valve rocker arm is worn, it is necessary to check whether the working surfaces and working heights of the intake and exhaust cams on the camshaft are worn or damaged.

III. One-way check valves. Check whether the check valve plates and springs are damaged, and confirm whether the movement of the check valve plates is smooth.

Installation of the camshaft. When reinstalling components with friction, apply the specified lubricant and lubricating oil to avoid initial wear of the components;

I. Assembly of the camshaft rocker arm bracket. The side with the "EX" mark on the camshaft rocker arm bracket is the exhaust valve rocker arm. Install the one-way check valve, intake and exhaust valve rocker arms, and valve rocker shaft. The bevel at the end of the intake side valve rocker shaft aligns with the bolt on the camshaft rocker arm bracket, and the bevel at the end of the exhaust side valve rocker shaft aligns with the bolt on the camshaft rocker arm bracket.

II. Installation of the camshaft rocker arm bracket. Rotate the magnetic motor rotor so that the "T" mark line on the upper stop point of the magnetic motor rotor aligns with the timing mark on the engine block. The circular hole on the camshaft timing transmission chain wheel is upward, and the two concave points on the left and right are parallel to the contact surface of the cylinder head (the camshaft cam is downward in a downward eight shape). Install the timing transmission chain on the camshaft timing transmission chain wheel, and install the camshaft on the cylinder head. Install the fixing pin, camshaft rocker arm bracket, flat washer, and nut on the cylinder head. Apply some oil to the threaded part of the camshaft rocker arm bracket bolt. Tighten the camshaft rocker arm bracket bolt in two to three turns alternately (torque value: 20N).

III. Adjustment of the camshaft timing transmission chain adjuster. When rotating counterclockwise, the locking is released, and the timing transmission chain is tightened. Install the timing transmission chain adjuster cover screw back.

IV. Adjustment of the intake and exhaust valve clearance. Rotate the magnetic motor rotor so that the "T" mark line on the upper stop point of the magnetic motor rotor aligns with the timing mark on the engine block. The circular hole on the camshaft timing transmission chain wheel is in the upward position, and is perpendicular to the contact surface of the cylinder head. The camshaft is equipped with a pressure-reducing reverse rotation device, so the crankshaft cannot be reversed during valve adjustment, otherwise the exhaust valve cannot be adjusted. The working clearance of the intake and exhaust valves is 0.12mm. When adjusting, loosen the locking nut, rotate the adjustment screw to make the valve working clearance meet the technical requirements. After locking the locking nut, check again whether there is any change in the valve working clearance.

V. Replace the cylinder head gasket, use as little sealant as possible, install the cylinder head cover, and tighten the cylinder head cover fastening screws.

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