Get rid of the Zinger and try an MSC, Menz, 3w or even a Mejlik. These props are twenty times better than the zingers simply because they are much much stiffer and have really good effieciency numbers. They cost about 5 bucks more (except for the Mejlik) but they are worth there weight in gold to a Zinger.

Your rpm numbers will drop greatly but the overall thrust will drasticaly increase. You would would not believe how much of a difference a prop can make on these gassers.