Heres a few things ive done to my 1.6 carb thats helped:
1) where the carb connects to your left front 1/4 pannel theres a small pad inside that restricts the air, RESTRICTING it being the opperative word, Remove that and fit a small square of wire gause inside the hole instead, now reattatch your carb
2) Clean your throttle boddy and carb using carb cleaner!
3) Dont just bolt on a muffler to your piping, Get your self a whole new exhaust kit from the manafold back
4) Uprated spark plugs and leads often do wonders
5) oil change
6) Giving the car a few good thrashings to the redline and back can sometimes clean out the moter nicely
