If these are the foam air filters we are talking about.....You will find that any air filter WILL create drag of some sort, and so the mixture will richen. You may find you need to lean off the idle adjustment a bit, and possibly go smaller in main jet size. But you can only find this through experimenting around, and checking plug colour. MartynG did some exhaustive tests in this way with his new Amal Premium MK 1 concentric carb on his Comet, and has arrived at a good result. An engine that runs quite rich over most of its throttle range will generally benefit from a slide with a larger number, say a 3 1/2 to a 4. A jump from say a 3 to a 4 may be too large a change and result in a lean condition instantly recognizable by spitting back through the carb. It takes a little time, but alter things one at a time and check as you go via road testing.............Greg.