A number of people on this forum have had issues with these engines running unreliably. The theory is that the carburetor configuration puts the carb directly in the prop blast which can result in pressure fluctuations across the diaphragm. Some people have fixed the issue by soldering the hole in the diaphragm cover closed, installing a new brass nipple fitting into the cover, and running an extension hose into the fuse to keep the pressure at atmospheric at all times. Others swear that this is just putting a band-aid on bad tuning. Just trying to give both sides of the story. Here's a good thread to check out on this issue when you have time.

http://www.rcuniverse.com/forum/gas-...as-engine.html