market dynamics
Valve leakage in motorcycle maintenance practice
Release date:2024.01.25

First, the valve leakage detection method:

Method 1:

First of all, the cylinder pressure is insufficient, and the starting rod is not stepped down too quickly a few times to feel no resistance or little resistance. If the cylinder pressure is insufficient, it indicates gas leakage, but not necessarily valve leakage. First check whether the spark plug is tight, and if it is not tight, check it again. If the cylinder pressure is still insufficient, unscrew the spark plug, pour 10 ml of clean oil into the spark plug hole, tighten the spark plug, and then test the cylinder pressure with the starting rod several times. If the cylinder pressure is enough, it is the piston ring leakage; If the cylinder pressure is still insufficient, determine that the valve is leaking.

Method 2:

1. First remove the spark plug, exhaust muffler and hose or screw on the intake pipe. Please note that some cars are equipped with process screws on the intake pipe to detect the negative pressure value of the cylinder, such as Honda CB125T model; The intake pipe of the scooter engine has a rubber hose leading to the negative pressure switch, such as the Honda CH125 water-cooled scooter (Black shark model), Honda CHA125 air-cooled motorcycle (GY6 engine); In the market, there is no such structure on the intake pipe of the model with the plunger carburetor of Euro Ⅰ motorcycle, as long as the air filter is removed (Note: The intake pipe of the motorcycle with Euro II and Euro III is provided with an intake joint). For Euro I models, after removing the air filter, wrap the plunger carburetor inlet (that is, the joint with the air filter) with a plastic film of the appropriate size, and poke a thin hole in the middle of the plastic film with a small needle to facilitate observation or hand test if there is no gas emerging.

2. Rotate the crankshaft slowly according to the working direction of the engine, so that the R arc surface of the valve rocker arm gradually leaves the camshaft lift until the test cylinder piston is in the position at the end of the compression stroke (as shown in Figure 6), that is, the test cylinder inlet and exhaust valves are closed. The "T" mark on the flywheel rotor can be aligned through the left cover timing inspection hole mark, or the cylinder cover can be opened to confirm the engine valve timing, so that the intake and exhaust doors are closed. Then hold the special sleeve plate to fix the bolt at the left end of the crankshaft, so that it remains stationary (two people cooperate when conditions exist).

3. By adjusting the carburetor throttle screw, the carburetor throttle is basically closed


4. Hold the rubber hose with the ventilation pressure of about 3 ~ 5kg/cm2 to the spark plug screw hole of the cylinder cover, and try to keep the joint without air leakage.

5. According to the joint on the cylinder cover exhaust port and intake pipe (or the plunger type carburetor inlet plastic film in the middle of the hole) whether there is a gas leak, you can accurately determine which cylinder (refers to the multi-cylinder machine, can be detected according to the timing position in turn) and which valve is not tightly closed and leaking. Thus, the valve and valve seat can be inspected in a targeted manner and repaired or replaced as appropriate. It must be pointed out in particular that before using this method, please check whether the engine valve timing and valve clearance are in normal condition to avoid misjudgment.

6. If the valve or cylinder head and cylinder body joint surface are not leaking, the engine crankcase rubber vent hose port can be immersed in the prepared small water cup, if there is continuous bubbles, indicating that there is an obvious leak between the cylinder and the piston and the ring. In order to verify this fault, you can remove the spark plug, pour an appropriate amount of lubricating oil with large viscosity into the cylinder, and after a few minutes, wait for the oil to slowly penetrate the gap between the piston ring port, and form a thin oil film between the cylinder and the piston, and then test according to the above method. If there is no continuous air bubble in the crankcase vent hose, it can be confirmed that there is a leakage fault at the cylinder and piston ring.

Considering the difference between the cold and hot state of the engine and the seal leakage, it is recommended to test separately under the cold and hot state in order to compare and avoid misjudgment.

Solutions for valve leakage:

1. Check whether the valve gap is too small, resulting in air leakage

2. Check whether the intake valve, exhaust valve and valve seat are normal

3, check whether the valve rod deformation, bending, wear, carbon accumulation, ablation, damage.

4, check whether the valve spring is back

5. Check whether the valve oil seal is normal

Method of grinding valve:

Grinding valve is a technical work, many masters will not teach you, because there is a lot of knowledge, technology, is not open, so that you do not make money. However, in line with the purpose of learning together, I would like to share it with you and hope that you can understand.

Grinding valve first prepare an intake valve, a tubing, valve sand (thickness), hand drill.

Now the cylinder head and valve seat are removed. Check and find that the intake valve due to wear caused by leakage, then we put the prepared intake valve into the intake valve seat, insert the oil pipe into the valve rod, one end of the valve rod, one end of the electric drill, put the valve sand (thick) on the gas door, and then start the electric drill rotation, it is best to use a slow speed, because good things are abrasive, fast is not good, after grinding 5-7 minutes, check whether the valve is abrasive. Then grind with valve sand (fine) for 3-5 minutes. When replacing the valve, be sure to pay attention to the need for professional tools and professional personnel to replace it.

Inspection method:

Put the intake valve and exhaust valve into the valve seat, and then turn the cylinder head upside down, add water to the intake port, and see if the water leaks out, or see if the light leaks out.

What to note:

In the ventilation valve, be sure to clean up the carbon inside, and clean up the carbon on the top of the piston. And replace the valve oil seal.

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