market dynamics
Motorcycle maintenance sharing
Release date:2022.12.22

Recently received a Dayang DY125 scooter, acc ording to the owner described his engine idling unstable, easy to stall. In some repair points also checked many times, because the failure sometimes did not, has not been completely solved.

Check after picking up the car, turn on the ignition switch, press the start button after the engine starts more smoothly, after starting the feeling of idle operation is not stable enough, stand for a while idle speed slowly decreases and self-flout, check and confirm that the engine has a bad idle speed fault.

After repeated adjustment of the mixture adjusting screw and throttle adjusting screw of the carburetor, the fault could not be removed.

The cause of engine idling failure is more complex, ignition system, fuel system and engine work abnormally, will lead to engine idling poor operation.

Remove the high pressure cap from the spark plug and screw it off. Hold the high pressure wire close to the cylinder block within 1CM and press the start button to check the spark strength. The high voltage ignition spark is generated between the high voltage wire and the cylinder block. The test confirms that the ignition system is in good condition.

The starting rod is pressed hard again to check the engine cylinder pressure. When the starting rod is pressed, the stepping resistance is not very large. The cylinder pressure meter is used to measure the result that the engine cylinder pressure is a little low, but not low enough to not be able to start.

Is the cylinder pressure caused by tight valve leakage? So open the engine valve chamber cover, check the working clearance of the engine inlet and exhaust valves, and confirm that the valve clearance meets the technical requirements. It seems that the reason is not here.

Then wash the carburetor, decompose the carburetor for cleaning, checking, no abnormalities found, after cleaning and reinstall the carburetor, at the same time adjust the carburetor mixture concentration is appropriate.

Then start the engine for inspection, the idle condition of the engine is slightly improved, the idle running time can be slightly longer, but the engine will still stall by itself.

Thinking for a long time, I think this failure should be caused by cylinder pressure, but since the valve does not leak, it may be caused by the wear of the piston and ring for a long time. After consultation with the owner, the owner is not ready to replace the cylinder, said that the car is old, do not want to invest too much money, encounter such there is no way, then from other aspects to see if you can make up for this problem.

According to the principle of ignition system, the working gap of spark plug electrode has a great relationship with the compression ratio of engine.

It can be understood that when the working voltage of the ignition system is a certain value, when the compression ratio of the engine is larger, the electrode working gap of the spark plug will be smaller. When the compression ratio of the engine is smaller, the electrode gap of the spark plug can be larger.

That is to say, the spark plug electrode working gap hours, the required ignition voltage is lower, the spark plug electrode working gap is larger, the required ignition voltage is higher.

This is because the ignition voltage generated by the ignition coil must be raised to a fairly high level before it is possible to break down the mixture and jump over the working gap of the spark plug electrode to produce a spark.

When the engine cylinder pressure is low, the ignition voltage has not risen to the peak, the breakdown of the spark plug electrode gap began to jump fire, the spark is not thick enough, the ignition energy is not strong enough, the ignition effect of the combustible mixture will become worse, especially in the idle engine condition, the high voltage ignition spark energy is small, the combustible mixture can not burn normally, the engine can not produce enough power, It causes the engine to idle unstable failure.

In the case of a decrease in engine cylinder pressure, you can try to adjust the working gap of the spark plug deliberately larger, so as to improve the jump-off voltage of the high voltage ignition spark, the energy of the high voltage ignition spark will increase, the effect of combustible mixture will be ignited better, the engine will improve the power, the idle operation of the engine will be significantly better.

Based on this principle and experience, I will adjust the working clearance of the spark plug a little larger, and then start the engine for test. After the engine accelerates, the throttle will be taken back, and the working speed of the engine will fall back to idle speed steadily, and the idle speed will be stable, and then increase the opening of the carburetor throttle to test the engine acceleration. The engine acceleration is sensitive. After several test runs, it is found that although there is still a flameout, but the overall has been significantly improved.

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