Some time ago, a motorcycle maintenance master took a lot of effort to repair a scooter, because he almost went into a detour because of an inadvertence of a small accessory. Today take out to share, in order to learn from.

The scooter was sent for repair because it was difficult to start, because the car was in older condition. After routine inspection of oil and electricity, no obvious fault was found. Because the cylinder pressure was low, the piston and cylinder were checked, and some strains were found, so the cylinder was replaced.

Unexpectedly, after the replacement of the cylinder, it was still not good to start. Once again, the cylinder pressure was measured and the value was normal, which was a little strange. Have to check the circuit again, press the electric start, measuring the battery voltage is 13.1V, the voltage drop is not more than 2V, remove the spark plug, stick on the cylinder to test the fire, blue and white sparks and snapping sound, ignition part is normal.

Connect the oscilloscope to capture the ignition signal and intake negative pressure, found that the negative pressure is too low, does the valve leak? Then check, and check the air tightness of the inlet and exhaust valves, and found no poor seal.

In the process of disassembly, it was found that the cylinder was not wet enough and there were few oil stains, indicating that the fuel was not enough to enter the cylinder. Re-manage the idea, the inlet and valve have been checked, there is no air leakage, then, what else affects the negative pressure of the intake? Oh, and a negative pressure switch!

General scooter fuel switch are negative pressure vacuum diaphragm type, when the electric start, the engine began to run without load, the negative pressure generated in the cylinder by the negative pressure switch will vacuum diaphragm valve open, the fuel into the carburetor, the vehicle can start normally. If the negative pressure switch fails, it will lead to poor oil supply. Apply negative pressure detection to the negative pressure switch with a medical needle, and it is found that only a small amount of fuel flows out of the negative pressure tube, indicating that the vacuum diaphragm of the negative pressure switch has failed.

Replace the negative pressure switch, start the engine a dozen, the original start difficult troubleshooting. After the owner left, I managed to open the negative pressure switch to check in detail. As expected, I found small holes on the diaphragm valve. It is these small holes that leak air.