I have an 85. Rebuilt the engine, runs great now............ The screws on the carb I found are for idling only. They don't affect the top end speed. I have found that the air cleaner box has to be just right, ie. not too much oil on the filter, securely closed (the lid), or the engine gets too much air, and engine tends to die out. Also check for plugged with carbon, muffler (a common problem). This tends to inhibit top speed.
Follow Ron's procedure for troubleshooting a two stroke engine, found in the Moped Army website forum. Highly informative. Give it a try.
Good Luck, Dave