Just put a new kit in the carby, new air filter, new plug. Gave it an overhaul because it died and wouldn't start. Before it died it had to run on 3/4 choke to get any high speed. Now I can get it to start and idle OK but on high speed it surges all the time and eventually stops. Starts again straight up but does the same. Playing with the choke doesn't help. The whole kit was used - needle, mesh screen, diaphragm and gaskets. Where do I start on the troubleshooting trail?