hi my friends son has a aprilia sr50r factory and it wont start ,it has a good spark and the fuel is getting through to the cylinder but fouls up the plug and will not start .ive removed the carb and fully cleaned and checked the jets and all looks fine,the vacuum pipe from the head to the petrol tank is ok and when trying to start it fills the carb up with fuel.if I try and start and hold back the throttle fully the bike backfires,the plug is brandnew as well .any ideas guys please ??? thanks gaz