Scooter: Motor goes off when accelerating

Based on my limited knowledge on scooters courtesy of owning one for about 3 years and assuming your scooter is carb and has similar mechanism (BTW not able to Google your make and model it's rare)

• First of all check the carburettor. If the carb is blocked then you will notice the symptoms which you have mentioned , blockage at higher throttle.
• Check the Air filter: if the air filter is blocked then you will encounter similar issues.

The above two are very easy and basic checks which can be performed by a road side mechanic or by yourself. Should not cost much including labour.

In case your vehicle is still stalling after all this then I would suggest taking it to a proper mechanic and getting it serviced. As for my experience with scooters , they dont cost much in my country (India) to get serviced.

Also, if its a 4 stroke scooter then there are chances of the choke not being there in the vehicle , some scooters dont have chokes, mine had one but I have seen without chokes.

What is the most probable cause for this?

In the most general terms, the engine isn't running because there is either too much air or too much fuel.

Can the cause be that the machine was not in use for some months?

Is it true that petrol may "become old" after some month so it does not burn anymore correctly?

It is possible, especially if the fuel didn't have good-quality stabilizing additives in it.

How high is the difficulty to repair this (clean anything inside, change spark plug) or do I need a garage for that?

This depends on what the problem turns out to be.

Does anybody know where I can get a service manual for that model?

Google is not likely to help you since its services are not permitted in China. The source for such information is also unlikely to be readily available outside of China.

It really sounds like your main jet is clogged. You can loosen the hose clamp holding the carburetor, then twist the carb a little to break it loose from the intake. Take pictures or tag all hoses and electric choke wires then remove them from the carb. Lay an old towell or white rag on your work bench. Turn the carb upside down and remove the float bowl. In the center of the upside down carb you'll see the main jet (brass cylinder). Use a clean strand from a bicycle brake cable an clean the hole all the way through the main jet. You'll have to hold the sliding cylinder so that the carb throat is open. This is so that the strand of wire itself passes into the carb throat. Use WD40 in the jet in case its really dirty. Carb cleaner will destroy rubber seals and the rubber throttle slide skirt inside the top of the carb. Carefully reassemble the float bowl and reinstall the carb and its hoses. Good Luck.

As per my understanding, the auto choke is a biggest blame. Disconnect auto choke cable which is going into carboraror solenoid. Try it