market dynamics
745/5000 It is not good to consider the oil switch when starting the fault
Release date:2021.12.23

Some time ago, maintenance of a pedal motorcycle cost a lot of effort, an inadvertent small accessories almost let maintenance into a detour. Take today to share, to make reference.

The scooter was sent for repair because it was difficult to start. Because of its old condition, no obvious fault was found after routine inspection of oil and electricity, but the piston and cylinder body were checked because the cylinder pressure was a little low, and some strain was found, so the cylinder sleeve was replaced.

Unexpectedly, after the replacement of the sleeve cylinder, it was still not good to start. When the cylinder pressure was measured again, the value was normal, which was a little strange. Had to check the circuit again, press the power to start, measure the battery voltage is 13.1V, the voltage drop is not more than 2V, remove the spark plug, stick on the cylinder block to test the fire, blue and white sparks and crackling, indicating that the ignition part is normal.

Connect oscilloscope to capture ignition signal and intake negative pressure, found that the negative pressure is too low, is the valve leakage? Then check, and check the air tightness of the inlet and exhaust doors, and find no bad sealing phenomenon.

In the disassembly process, it was found that the cylinder was not wet enough and there were few oil marks, indicating that the fuel was not enough to enter the cylinder. The inlet and valve have been checked, and there is no leakage. Then, what else affects the inlet negative pressure? Oh, and the negative pressure switch!

The fuel switch of general scooter adopts negative pressure vacuum diaphragm type, when the electric start, the engine starts to run without load, the negative pressure generated in the cylinder opens the vacuum diaphragm valve through the negative pressure switch, and the fuel flows into the carburetor, and the vehicle can start normally. If the negative pressure switch fails, it will lead to poor oil supply. Negative pressure detection is applied to the negative pressure switch with a medical needle, and only a small amount of fuel is found to flow out of the negative pressure tube, indicating that the vacuum diaphragm of the negative pressure switch has failed.


After replacing the negative pressure switch, the engine was started as soon as it hit, and the original startup difficulty was eliminated. After the owner left, I tried to cut open the negative pressure switch for detailed inspection, and sure enough, I found a small eye on the real diaphragm valve, which is the small eye of leakage, not only let the oil efficiency greatly reduced, part of the negative pressure of the engine will also leak from here, so that the negative pressure in the cylinder decreased.