As you look in the engine bay around where the coil pack is there are few rubber hoses Breathers, Fuel pipes and water pipes. There is a T shaped rubber breather, its probably leaking air in and weakened the mixture. This is your ploblem im sure of it, But even if it isnt your problem, its very common for these engines to do this, so its worth replacing anyway before it starts giving you trouble, you can find out if its leaking by spraying a quick toot of carb cleaner around the area and if the engine revs increase then its leaking.
loz