the easyiest way to see if its an air leak is to buy a can of carb cleaner. with the engine running spray around any pipes and inlet manifolds. the engine will revv faster as it draws in the vapour where the leak is.

the other obvious one is the fuel filter, a dirty and/or block fuel filter can make them run like a bag o shite, have a look under the spare wheel well, if its rusty - its not been replced in a good few years