Scooter stalls when driving uphill?

Hello,

I drive a Sym Red Devil 50, built in 2007, 2-stroke, identical to a Sym Jet Euro x, built in 2007.

My problem is that the scooter stalls as soon as I go up a hill.

When I give it full throttle, the engine speed drops relatively quickly and it doesn't do anything anymore.

If I drive at half throttle it happens sometimes, or not at all.

I'm driving without an electric choke! Because it's broken, but the problem has occurred before, just as it has now.

I'm at a loss as to what it could be…

Thank you for your support