I just signed up here because I just got my 1981 XL185S roadworthy. I rode it to work today on the main roads for the first time. It would run great through the first three gears, and then bog at 45-50MPH. I pulled over and I figured out the bike doesn't like rolling the throttle in 4th gear with the choke on.... duh. Took off the choke and it ran perfectly from there out. (I even passed someone on the expressway.) I wonder if yours could be the same: It's getting too much gas too soon. Have you looked into adjusting the carb? BTW: My FIL has a '76 XL250 he babies, and my BILs abuse their early-80s XLs (250 and 350). Never had a problem we couldn't fix. They're great bikes.