Just to state the obvious, when you say you are running it for short periods, you do mean riding not just running stationery, when it will overheat. As Goldie says check the ignition timing. If that's ok look at the spark plug. If it is very pale in colour or in severe cases, burnt, then the mixture is too weak. It can be made rich by screwing in the air screw (to reduce the air) If this is not enough then raise the needle by one notch ( to allow more fuel).
All the best.
