It's not going to be an air leak. An air leak would typically manifest itself at low throttle settings.

Some suggestions:

1) Get another idle bar plug like the one you already know works well.

2) Try other plugs as recommended by other posters above. I'd also recommend an OS Type F plug - no idle bar but it is more resistant to flame-outs than many plugs.

3) If you really just want to fly, get a new engine that's made in Japan (OS, Saito, YS, Enya) or Taiwan (Thunder Tiger) and avoid the Chinese stuff entirely.