Hi there! My wife has the same problem on her 535 and its actually her third 535 so i know they can run up to at least 90 mph. I checked that there is sparks coming on both plugs and could not find anything strange in the carbs so i hope someone can help us with this. Its easy to start but misfires a little. At slow speeds you dont realise there is something wrong but when you try to speed up nothing really happens after 1/4 turn on the gas!?!