There is obviously a propblem with yours. My a14 with cam etc revs to 7000rpm unbalanced without any vibration. Even my old/worn stock a14 would rev to over 6000rpm without vibration. If you think an a12 will beat a a14 you must be kidding. The difference the extra 200cc makes is incredible. Bolt your gx head onto a a14 (modify oil galleries as required) you will notice a huge difference and it will save you from having to put a big cam in with the lumpy idle that you say do not wish to have.