Just had a new top end put in bike. Son took it to ride yesterday. Bike fired up easy rode it slow for a lap. When he got on the gas bike just cut out. He could not get the bike to start back up. When I got home kicked and kicked and finally got the bike to fire up. When I wacked the throttle the bike cut out. Took it back to shop today and got the bike to start. The mechanic rode it and when he got on the gas the bike died. Any ideas what might be the problem?