Well I run a stock airbox,filter and stock pipes on both my 18's but the lid has 4 25mm holes drilled in it,I didn't need to rejett because with the increase of air that is now going in the airbox and moving through the carbs will draw more fuel off the main jett's, you can use smaller main's but how much smaller I dont know and if I get it wrong then it will go pop so i'm happy with it as it is