Hi Steve, my reply might not help much. I bought a carb off a guy that fitted a S&S to his 95 fatboy. I intend replacing my 87 keihnin with it. I noticed it has had a large hole (1.5" high x 2.5" wide egg shape) cut into the back ie facing the motor at the bottom of the backing plate.

all you will get is a whole lot of hot air which aint good, in a carby bike this may cause pinging, with an efi bike it will cause ignition to be taken out, between maybe 5 and 10 deg, and will hurt performance gains achieved with more flow... fit a decent hi flow n dont waste the time with the drill

get a high flow unit , drilling holes into the back plate will most likely do worse for perfomance than good ..

the standard SE hi flo doesnt have a normal backing plate the chrome front cover mounts to the filter so the whole back half is open