I have an 1980 express which behaved the same way. The engines on these little buggers are very sensitive to the condition of the air cleaner foam. Not having an original foam piece I used a green foam element for a B&S lawnmower. Up to 23 -24 MPH everything was fine. Above that speed it would bog down and I would have to back the throttle way off and then slowly increase it again. I decided the foam was getting sucked into/against the rubber air tube and choking off the air. I repositioned the foam to get the hole in the center more centered and I got my 30MPH back.
I have sinced switched to the grey, more porous, foam with out any problems.