How can I fix my T50 mitsubishi brush trimmer. ?

The machine is a professional brushwood trimmer approx 10 years old. It starts with difficulty then will run for a minute or two then cuts out. Then it is hard to start again. I have renewed the carburettor also diaphrams and gaskets and a new plug which has excellent continuous sparking when held against the engine. The fuel lines are clear and I can see the petrol is flowing through the plastic tubing to the carburettor. Have checked for air and petrol leaks on the carburettor which there are none.