In my case of low RPMs with my 30cc I was hoping the piston was installed incorrectly but found it wasn't. I can't get over 6900 with a 16x8 prop even with a dual outlet muffler. I tried an 11mm carb on the stock intake with the dual outlet muffler and still 6900 RPMs. Those "R"s wouldn't fly the plane I want it to. I have since opened up another intake to match the 11mm carb but haven't tried it yet. I'll post results when I get to try it.