Race type airfilters feature a significantly larger surface area than the original ones, thus reducing suction resistance (a similar effect may be noticed when trying to breathe through a straw as compared to breathing freely). This translates into more fuel-air mixture reaching the combustion chamber. The mainjet of the carb has to be changed to a larger size to make sure the fuel-to-air ratio remains the same, thereby preventing the cylinder from overheating due to a lean mixture. The downside of this modification is the slightly increased noise level. Polini airfilters come in a choice of 4 different shapes and 3 different connecting diameters.
36mm will fit Dellorto carbs from 12-17mm, Amal from 17-21. 32mm for Dellorto 19-21, 39mm for Dellorto 24-26. The Axwin airfilter is long and straight and will fit any Dellorto and Amal carb from 12 -21mm with a universal adaptor.