I have two GMS 47 engines and have different problems every time I switch them to a different plane. I know you pulled the pressure line and it continued to run, but that does not account for changing throttle. The motors with the smooth muffler do not always supply enough pressure, and this is somewhat dependent on tank location. Some people have added baffles to the muffler inside to solve the problem. I have found simply enlarging the pressure fitting in the muffler with a number 50 drill bit has given a more consistent throttle response in all directions. I also have good luck with morgan fuel, 15%. I do run the # 3 plugs.