I would say two 1/2" outlets are a touch too small for that size engine for optimum performance. If noise isn't a problem, I would add more outlet. However, you also mentioned that you weren't sure you were getting full throttle, or maybe the mixture wasn't quite right at full throttle??

Before I installed my little twin cylinder job on my Giant Stik, I had my Gas/Glow Poulan 42cc on it. It is fairly heavy, with heavy duty servos, a smoke system, large tires, etc. With the Poulan, it would roll about 30 feet on takeoff. I could then pull it into a 45 degree climb while doing a roll at about 20 feet off the ground, and then I could pull it vertical. It would then just keep going until I couldn't see it anymore!

Another thing is the prop pitch speed. Many, if not most of us transitioned to gas engines after flying glow powered planes. The glow engines tend to rev a good bit higher than their gasoline counterparts. So consequently when you go to gasoline engines, you have to use more pitch in the prop along with a bit smaller diameter than "normal" to achieve similar performance. A lot of guys (to me) seem to over prop their gas engines. I ran an 18 x 8 prop on my 42cc engine, which many would consider way too small. However, I really liked it's performance. I like to "wing" my engines a little higher rpms than most, but they seem to like it. Keep in mind though, that I don't fly 3D, and I do enjoy a plane that I can cruise around with at half throttle, and nail it for some blazing speed and/or conventional aerobatics when I care to do so.

AV8TOR