I've put 15,000 miles on my sporty and have also put quite a few miles on my old man's EG. Whether it is the evo or twin cam, these engines don't like low rpm's. I learned that lesson very quick when I got my sporty 3 years ago. I highly doubt it's something wrong with your belt. You just need to get used to how this bike should be rode.

 

The x14ied's do smooth out riding at different speeds for sure.

 

There is nothing wrong with your bike. My 07 1200C does the same thing as yours. I took it back to the dealer because I thought something was wrong with it. I had the stage 1 done and it improves some but it still as some of the jerking feeling in 1st, 2nd and third unless you get on it pretty hard. Just ride it, you'll be fine.

 

I pretty much eliminated bucking and farting on my 883 when I went up a size on the low jet (even with stock pipes), but the instant acceleration/deceleration with minute throttle movement at low rpm still exists. Not a problem with nobody in front of me, but following cars from a stop requires a riding style I don't like; let the car get 2 or 3 lengths ahead, accelerate to about 10mph, pull the clutch and coast, when the car pulls ahead enough, do it again, and so on till I can finally get into second gear; usually it's only a problem in first. Taking half a block to get enough speed for second isn't unusual the way so many people drive cars today. I've driven miles in city driving without getting above 3rd - and gotten terrible gas mileage for it. I may get slammed for saying this, but for doing a lot of city/slow traffic driving, a large (at least 250cc) scooter is way easier to ride than a big bike, if you don't care what people think of you. Maybe I should get that Helix sitting in the back of the garage running again...