My brother's first car has just been put on the road this week after being off road for a few year while welding was done etc.

When we took the car out everything seemed fine locally at around 30mph but once we went to around 60 the car started to splutter, flat spot when accelerating and lack of power aswell as it cutting out etc.

The engine is a 1.4cvh carb, it seems fine when you first drive it but after about 10/15 mins the above happens, any ideas?