Tell us more about the type of fuel system used and the year of the car and I might be able to help. I'm presuming this car hasn't got a cat. so if it passed the Mot it must have a C.O. content of less than 3.5 percent.The car probably has an air flow meter on it. This is located near the air filter. It will have an allen key, air bye-pass screw near this. This needs to be turned to adjust the fuel mixture. Don't know which way, can't remember! It needs to be put on an emmisions machine to set it up. It's no good just guessing. If it has a different fuel system on it, it will all be different.Some have electronic C.O. (carbon monoxide)adjusters.We need more info to tell you more. Easiest thing to do would be to have it tuned at a garage of your choice.