I had the exact same problem with mine. I replace the cdi and changed it over from DC to ac using a conversion kit I got from a shop in Canada but still had the same problem. A friend came over one day and we got to talking about it and he was like can I take a look, sure why not. He took the carb out and drilled out the brass plug for a needed that was underneath of it. Made an adjustment to it and put back in. It ran like it was new. No problems since, 70 mph, and felt like it even had a little more get up and go from a dead stop. Hope this helps, I could have saved a lot of money and time if he would have stopped over for a beer sooner lol.