Welcome Kymco Owners!
Just to be sure: has your idle screw o-ring and washer (pls see pic)And: is your bystarter working properly, means new O-ring and gasket?One more thing came in my mind:-the valve needle within your carb tends to wear out on top, means, if this little washerlike piece is below one millimeter, could cause such behaviour.
Here is your other tread.https://www.kymcoforum.com/index.php?topic=33452.msg234964#msg234964Q: do you have somewhere on your carb a idle screw which looks like the one pictured by me?See link https://www.kymcoforum.com/index.php?topic=33440.msg234944#msg234944If NO, likelyhood you lost it from this hole, is highIf YES, we have to search further