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Not knowing how to use an oscilloscope when repairing a motorcycle
is really frustrating when encountering problems
A GY6 clone owner reported that sometimes the motorcycle does not accelerate. Since the user does not understand vehicles, they could not provide a description that would help diagnose the fault, so they left a phone number for me to assist in troubleshooting.
During a road test, I found that the motorcycle occasionally hesitated at speeds of around 70 to 80 km/h. Based on past experience, I thought it might be an electrical issue. I suspected that it could be a misfire, which is sometimes difficult to detect visually. Before having an oscilloscope, I used to rely on replacement methods to troubleshoot issues. Now, of course, I use an oscilloscope to observe the secondary ignition waveform. While holding the brake and applying some load, I found the waveform to be normal.
Next, I used the pressure waveform to check. This pressure sensor reflects different voltages under atmospheric pressure depending on the weather temperature. Today is very hot, and it reflects a voltage of 5.6V under atmospheric pressure.
At idle, it is clear that the gas phase timing is abnormal.
Looking at the waveform during the hesitation, I was even more certain that the fault was in the engine. Doesn’t it look a bit like a waveform caused by exhaust blockage? That’s what I thought at the time, but later I checked and found that the exhaust was new… If this waveform persists after removing the exhaust, I would also consider the camshaft issue.
After diagnosing, I called the owner and informed them that the engine needed repair. After getting their consent, I removed the engine to check the tensioner, which still had one-third of its adjustment left.
At the same time, I checked the intake valve, which had zero clearance, but the gas timing was half a tooth late.
Upon removing the camshaft, I found that it had already worn down, but the wear was very even, so there would not be a lot of noise. What puzzled me was that the small chain was not loose, so why was the gas timing half a tooth late? If anyone knowledgeable could help discuss this, please leave a comment below.
Since I had disassembled it, I replaced it with a new cylinder. When reassembling the camshaft, I found that no matter how I installed it, it would either be half a tooth late or half a tooth early. I preferred it to be early, so I reassembled it half a tooth early.
After assembling, I started the engine, adjusting the mixture while observing the waveform, and cleaned the carburetor thoroughly.
After adjusting, I conducted a road test and confirmed that the fault had been resolved.
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