Welcome to TeamKitfox Brannan. I also doubt the carb ice just yet and also suggest a close scrutiny of your entire fuel delivery system. With the info. provided it sounds like fuel delivery restriction. If the aircraft is brand new check your fuel filter/filters for contamination from fiberglass from the fuel tanks. Make sure your are flowing enough fuel at the engine to maintain full power for extended periods of time. Your engine will consume between 7 and 8 gallons per hour at full throttle. You should be delivering a good percentage over that for safety. 50% over max fuel usage should not be at all hard to acheive sitting on the ground in any configuration. That is of course with no ram pressure at all from the fuel cap tubes. If not look for any restrictions and that goes for fuel lines that may look normal from the outside. If you are useing the old fuel valve supplied with many of the older model 4 kits I would inspect that real close. The old models used o-rings inside and they are susseptible to damage and blockage of fuel flow.

Good luck and please let all know if you find the problem. Be careful in the meantime.