First, simplest, try opening the plug gap to .025. Sounds crazy, but you might be surprised at the results. That move alone cures much of the issue on many of them almost completely! Same trick works as well on the engines in the 30-35cc class as well, and there is absolutely no downside of any kind. It works. It's free. It's easy to do. Try it if you haven't already.
After that, the mid range burbling is caused by a rich low speed mixture. That same rich low speed mixture allows the engine to be started easier, all but eliminates the potential for the engine to quit in flight, and reduces or eliminates an issue known as "high idle hang" that is often seen in 2 cycle engines that are tuned right on the edge (no matter what size displacement or how many cylinders they have!). When the throttle is pulled back to idle, the engine "hangs" at a high idle for several seconds, then comes down to what you expected it to come down to (really disturbing on final approach).
Unless you are one of those that enjoys endless tinkering with your needle settings, suggest you run it as is, burbling and all, and fly the heck out of it! It's a much friendlier engine all around when set that way....
Last thought on "burbling". Note that the engine is generally "coasting" when this is happening. There's very little load on it. Pull up or bank hard into a turn and that burbling will stop immediately. If it's really making you crazy, you could load the engine up with a bigger prop or more pitch. Problem doing that is you'll likely be affecting your performance for the worse. -Al
Last edited by ahicks; 07-13-2014 at 05:37 AM.