Vespa 50FL

Having problems getting this machine to run properly. Looking for suggestions and a solution.

The engine starts and runs fine but it will not idle it will die out or unaided rev up hold a high idle for a bit and the slow  down usual to a stop. Just put on a new carb and it still does it. I can get it to idle for a bit rev it up and of it goes doing its own thing. Engine and transmission has been completely rebuilt using all new gaskets and seals. new piston and bearings fitted.

Driving me crazy any suggestions