As Jimnie mentioned, it's a sad fact that some ARF's are designed for an inverted engine and the fuel tank is in the wrong place. There's not much you can do except re-build the whole front end, which cancels out the reason a person buys an ARF in the first place. How bad does the center line of the tank offset the carb spraybar on your plane Cambo? Anyway, a good starting point for the idle adjustment on an air-bleed is to adjust the needle until the air orfice is half way closed by the needle. If it was mine, I'd be tempted to cut a hole in the cowl and flip the engine over.