The carb on the GX 31 is small for a reason. I'm sitting here with a GX 31 intake valve between the jaws of a dial caliper. 12.5mm diameter. Their valves are TINY!!! For contrast, the intake valve on a 35cc Stihl four stroke is about 16mm. That tiny GX31's rotary barrel carb can supply all the air those little valves can handle.