RCS 1.8 Sputtering Problem
 

I have a RCS 1.8 installed in a Hanger 9 cap. The engine has about a 1/2 gallon run through it. I am having a sputtering problem at throttle settings greater than half when the plane is flying level or in a turn. The sputtering problem does not occur when the plane is going vertical at full throttle or on the ground. I have checked the fuel lines and the fuel tank clunck and found no problems there. I am not using a muffler. Any clue to what may cause the sputtering problem? Thanks in advance for any help