My GUESS is that he has you a little lean off idle. I have done that with my SEPST to my bike, and it is immediately evident. It doesn't take much to affect the pull.
On the other hand it will do the same thing rich. Seems less pronounced if I get too rich though.
...and I used to think my Sportster was heavy...
A Harley will get you through times of no sex, better than sex will get through times of no Harley.
SE Heavy Breather
V&H Monster Ovals
SE Pro Super Tuner to make the above play nice together.