i have a 78 peugeot 103 and i need some help. when it warms up and its gone a few miles it starts to sputter and not want to run at a high rpm right around 30 mph. now i just cleaned the carb put a new plug in and decarbonized the pisten. it is running great till it gets hot. what can i do..... i dont know if i can get a different plug or if thats the way to solve the problem.. any help would be greatly appreciated thanks..