I'd say you want to do a few things. Cleaning the muffler will be a huge improvement all around, even if you can already blow through it fairly easilly.
I'd also make sure your spark plug is the correct one and gapped at 0.026"
After that, check the needle on the end of the throttle cable. It's keeper C-Clip should be in the middle notch for most engines. Anything else will effect your mixture at higher speeds.
My Express would often stutter pretty badly when the carb was dirty or when I had a leaky O-Ring in the carb, but it does something similar now that my muffler is clogged. It also likes to act strangly if the spark plug gap is too narrow.