i have a derbi predator gp1 50cc liquid cooled moped 2003 plate and ive just put a tt leo vince exhaust pipe onto it , once i done this i had the bike running an riding for a good 10mins but there was lag in the power , once the bike stopped its not starting again , ive cleaned the air filter changed the spark plug fitted a new battery but still wont start , could ANYONE help me ?