Modern Vespa

I have some problems with my GTS150 (Vietnam model) 2014. I have ridden it every day for 5 years but I have some issues with it. The engine starts pretty hard in the morning and I cannot hit the top speed if ride over 100kph (60mph) it starts jerky after that speed. I replace lots of things: v-belt, spark plug, spark cap & wire (NGK), Battery, air filter, variator&clucth service, O2 sensor, ECU. These changes cannot help it better. It still happens until today, so annoying.

If anyone faces with the same issues with me please kindly suggest for the solution. Thank you so much.

Need more information. What happens when you let off the throttle? Is it engine related? When is the last time belt and rollers were replaced?

Greetings:

Sounds like power cuts out repeatedly "Jerky?" at higher RPM. Multiple possible causes - RPM limiting device is my first thought if transmission rollers undersized or worn.

Thank you for your kind attention. When i release the throttle it's always brackfire. The belt was changed 3-4 times after it happens. The roller is fine.

The belt is Polini i changed it last week. I think aomethink like that too because i install the power cable of NGK spark plug it may result high voltage of ignition. However, i found that the injector was not spray when i turn on the key. Is it abnormal or not?

I am not familiar with fuel injected systems. This is beyond what I can advise. If you have fuel injection you should have a check engine light. Is the light on?

Backfiring suggests a timing/valve problem. How many miles, and have the valves been adjusted? (Assuming this is a 4T engine.)

There is no fault code or signal light appeared on the dashboard.

The mileage is around 68,500km. I have been adjusted valves since 50,000km but this problem occurred before those miles. It doesn't help anything better.
I took the EFI cleaning yesterday but there is still having problem excepted a little bit easy start-up engine.

I concur. It's your valves. I have one of these scooters. Valves do require maintenance as per the recommended intervals.

Hmmm. Isn't the S 150 fuel injected?

The LX and S were carbureted up to about 2010. Apparently some of both in 2011, then FI from thenceforth.