One of the problems with using an RC carb wired wide open is that they're usually far too big for the revs needed for stunt so fuel draw can be marginal. This can be cured by trial and error gradually closing the carb until it runs right (and at a guess I'd say maybe 1/2-2/3 throttle would be close). The best way of course is to use a venturi either made up or adapted from some other engine. It's been found that for stunt work there's a certain ratio of venturi area to engine size that gets you right into the ball park.

For your engine a venturi of about .290 bore with a spray bar that's .156" diameter would be perfect. Letter drill size of either L or M or a 19/64" drill will all do the job. If you use a smaller diameter spray bar you'll have to reduce the drill size a little because the final effective area you want will be in the range of .02-.023 sq inches. If you're thinking about running without a muffler then you'll definitely need to use a small venturi size.