How many miles on engine? Original E5o one speed? What do you weigh?
All have impact in addition to everyone else's ideas. XK had the Magnum frame (almost identical to it, except for one pretty useless design difference near the bottom where the engine bolts on) and, depending upon your weight, age/mileage of engine, other stuff previously mentioned, I have heard of those going 25mph or less. Gotta admit, the Magnum was a fairly heavy bike if everything was still on it.
Do what everyone else suggested first (get rid of carbon everywhere), invest in a new spark plug (just because) and try it again.