hi there i have had my 05 zip sp for around 4 months. it is de restricted and does 60mph on the flat or 65-70 down hill. recently, i have had a few problems with it; my bike when cold runs fine, when it starts to get hot it makes spluttering noises and only reaces 40mph. i have cleaned the carb, drained the fuel, and changed the plug twice. please can anyone help me??? thanks