From our own Merlin, Dinqua:
Remove seat.
Remove snorkle and Air filter.
Remove any more snorkles from inside airbox towards carbs (not in Speedmasters).
Notice the bottom of the airbox has small dimples that hold the air filter in place. DO NOT DRILL THESE OUT, stay inside them as you do the mods.
Stuff some clean rags into the airbox so that plastic particles stay out of the carbs.
Place a piece of wood, we use 3/4" plywood, under the airbox on top of the swingarm as not to chip the swingarm podercoat when you drill through the bottom of the airbox.
Use a hole saw,(smaller than the distance between the oval dimples) or make several smaller holes and then dremel them into a larger hole on the bottom of the airbox, staying inside of the little protrusions that hold the air filter in place. (They are in an oval shape).
Then vacuum all the little pieces of plastic from the inside of the airbox and remove the rags. be sure you get ALL the little pieces. Plastic in the carbs is not a good thing and can easily gum up.
Then as an added precaution, we cut a piece of light guage stainless steel screen door material (or you can even use the plastic stuff too) into a slightly larger oval than the protrusions.
For added safety, and to prevent water entering the bottom of the airbox (which can happen but is extremely rare and only if you ride several miles on water logged roads), you can cut two equal sized pieces of screen material and place a thin piece of foam, like the uni filter foam stuff, between the two layers of screen material. You can also add some of the filter oil to this new bottom filter as well. Then bend the edges of the screen material over so it secures the layers of screen and foam.
Then take the screen piece, be it one or two layers, place it into the bottom of the airbox completely covering the oval dimples.
Cut the bottom out of your air filter but be sure to just cut the inner part out of the bottom, leaving the outer thick edges for grip in the bottom of the airbox.
Firmly press the air filter in place on top of the screen you inserted, it should "POP" a bit as the screen deforms and seats itself between the filter and the airbox.
Be sure the air filter is down far enough to reinstall the top holding plate for the air filter. If not, press it down harder, or use a less rigid stainless screen for the bottom filter assembly.
Replace the top part that holds the air filter in place.
Put the seat back on and ride away.
You will most likely have to rejet after this mod as it allows about 50% more air into your carbs and you will need to balance the gas to match.
This modification is easily reversable by simply replacing the air filter with a new one. The uncut bottom of the filter should plug the hole completely, if not, just cut a small piece of plastic/rubber the same size as the holes and place that under the new air filter, or glue it in for a better seal.
