The rumble is too lean on aircraft pilots lean out their fuel for better performance at altitude for fuel savings then forget to adjust to a richer mixture in lower altitude. That's why you will hear small airplanes fart and rumble on landing approach. The spacer under our Rochester is 3/8" in thickness. You might be boiling gas also we wrapped our fuel lines with heat resistance jackets. Gas filter maybe dirty and air filter? I was once told that 99% of all DIY mechanics go to the carb as ther first choice for trouble it turns out it's 99% chance that its not the carb but elsewhere...go figure.