I have an 05 Fatboy (88c.i.) I recently installed hi-flow air kit and I am thinking about installing a set of LAF Y Drag pipes. Any advice on what size main jet to switch to? (I'm assuming I should leave the stock slow jet in the carb)? Any suggestions would be appreciated.

Thanks

 

Actually the main jet is usually a tad big in a stock carb.
The EPA doesn't care about wide open throttle , so the stocker is a pinch fat anyway.
I can only relate to my findings on my last bike a 2006 Electra Glide Standard (carbed).
I had a #45 pilot and #180 main jet in this bike , stock.
I assume you have something similar in yours.
I added a set of V&H classic mufflers and an Arlen Ness Big Sucker to an otherwise stock bike.
I started with a #46 pilot jet and it helped the low speed mix and cold start.
I replaced the Main jet with an older sportster jet , 27094-88 which was a bit thinner than stock.
This made from idle to near half throttle richer and the fuel mileage and performance went up.
I then went with a 190 main jet and top speed fell off by 6-7 MPH.
Went back to the stock main jet.
I then went to a #48 pilot jet so I could get the mixture right at 1 1/2 turns out on the pilot screw.
I could only go this big because of the mufflers volume of flow at idle.
Here's a pretty good article on this.
http://www.americanrider.com/output.cfm?id=1022023
Unless you have to have them , drag pipes are a big nono for best performance.
Best of luck with your mods.
Mick