Modern Vespa

Ok. Here is a strange one.
Stock carb on a PX 200 E.
Stock filter, clean and tidy jets and everything as it is supposed to be.
The cab box seals perfectly and no leaks anywhere.

Turned the idle screw all the way in.
Turned the mixture screw all the way in.

Started rotating the mixture screw anti clock wise by intervals of 1/4 turn.

The motor idles perfectly with a meaty pop pop pop.

Each time I twist the throttle the motor stalls and dies.

Turned the mixture screw all the way out in intervals of 1/4 and tried every time to twist the throttle and managed to have the vespa running but it always stalls when i twist the throttle beyond 1/6 of the fully open position.

My spark plug is black and oily.

Works ok-ish with the choke pulled out.

I can not understand what is going on.

Help.

Thanks.

To rule out a clutch side oil seal air leak. Check the gearbox oil level and condition.

Maybe your choke mechanism is slightly open. The actual knob might appeared closed but you would have to remove the carburator lid and take a good look.

Thanks for the replies.
Swapped the carb, its fine now.
Did a part exchange with the old one plus some money, so everything is working as it should.