You are right about the pressure causing the engine to quit lean. I don't remember the brand, but I had to plug one outlet of a muffler on a Super Tigre .90. I have measured the pressure on a three-line tank by putting a long piece of tubing on the fill line and running it up a yardstick to make a manometer. Leave the tubing open at the top. All of the engines I have checked made a column of glow fuel about 16" high at full throttle. I used this info to make my own mufflers.
Chuck