Press the starter and the cranks get slower and slower

I have a hyosung sense. I turn the ignition on and then hold back brake and press the starter button. It starts to turn over normally but after 2-3 turns it slows down to nothing. Full of petrol and oil

 

I agree with accyuklad. You need a multimeter on that battery.
If it is more than two years old, could be time to replace.
But also check with your multimeter that the voltage increases when revving, so you know battery is being charged.

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I have got my battery charged at the garage and it is back in the moped. I press the starter and it goes like it should but the engine doesn't fire up

 

have you taken the bike off the side stand....

 

I am getting a spark and the side stand doesn't have a kill switch anyway

 

how is the motor oil? new? old? full? empty? and the fuel in the tank, is it very old?

 

Brand new fuel. And is the engine oil the same as 2 stroke oil? If so it was in the bike when I got it a few weeks back. So I have no idea

 

If scoot has been standing awhile with fuel in, the carb will likely be all bunged up, and will need serious cleaning.

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If it's a four stroke motor it needs a motorcycle engine oil of the right specs, not 2 stroke oil.

If it has stood with fuel in it for more than a month or two, chances are the fuel system, especially the carb and fuel lines need a proper clean through, even after putting fresh fuel in.

 

i would be checking out your top end next, could be dead rings. thought it was a 4t same as a kisbee, but looks like its a copy of minerelli motor....