There is a small hole in the air filter housing into which an appropriately sized allen wrench is placed. You don't have to remove the housing. The allen wrench is placed upon the fitting and pushed down and turned with the engine running. This will either enrich or lean out the air/fuel mixture. You want it as lean as possible. Too lean and it will stall. Too rich and it will not pass emissions. I have found the use of "Guaranteed to Pass" helpful.