Motorcycle Repair: sportster/1200/2005, yost power tube, slow jet

Question
I have what would seem to be a carburetor problem but may not be.The bike seems to have a flat spot just off idle.It coughs and spit thru the air cleaner when I give it some throttle.The quicker I twist it the worse it gets. Sometimes it stalls. I rejeted the keihin cv 40 carb with a #45 slow jet and yost power tube with a #185 main.The problem is still there. It starts,idles and runs well other than this. I don't think it's spark or engine related but it might be. this bike only has 600 miles in it. any help will be appreciated.

Answer
Three things:  First you  need to pop out the idle mixture screw at the back of the carb underneath and check the setting. You should gently bottom it and back it out about 2 1/2 turns.  Then you need to take a .020 needle drill and drill out your idle jet or replace it with a #48.  Then make sure your idle speed is at least 900 rpm.  Too low an idle speed magnifies the problem, but people love that bumpety sound of a super low idle.  I guarantee that your problem is the idle mixture just not quite rich enough.  Also remember Sportsters are notoriously cold-blooded so you have to let it warm up before nailing the throttle or you're going to the hesitation cough.  I fix this problem on Sportsters every day at the shop.