Wayne,
The answer to your question would be that you have possibly did not check the ring gap before reassemble. It should be around eight thow gap and when putting the piston in you should have the ring gap facing the exhaust port on a 125cc motor and on a 160cc should be facing the inlet port.
Now the carby valve, the normal setting is 'C' and it should be facing where the cut out is on the carby where it sits.
Fuel mix is 25 to 1 that is 25 parts fuel to 1 part oil or 200ml to 5lts.
Try this and see how you go.
Regards,
Bruce