In the case of my Sportster it took me a few rounds of trial and error to arrive at what I have right now, which is a well-tuned 883 that feels very good at all speeds.
I started with what most Sportster owners have been doing for years. I installed a Screaming Eagle air cleaner and HD low restriction mufflers. Since I do not like loud exhaust, I bought street legal "scripted" mufflers (they have Harley-Davidson on the rear muffler in script). I also re-jetted the carburetor to match what common advice was. That common advice states: assume 160 main jet as stock and go to 165 for either intake upgrade or exhaust and go with 170 if upgrading both. I upgraded both, so I had a 170 main jet ready. When I opened up the carburetor I was amazed to find out that my stock main jet was 175! This was one of several differences between the '04 and newer bikes and '03 and older ones. In 2004 the (HD) motor company redesigned the Sportster quite significantly. The major design change was rubber mounting the engine, which necessitated redesigning the frame, exhaust and other parts. Along with the engine mount change, MoCo also changed some cosmetics, brakes, ignition, final drive ratios and more. I also found out that the jetting was changed. I was stumped, but went with the 170 main jet. The bike ran OK, but I could not see the amazing results I was expecting. It still felt anemic. I then tried replacing the stock needle with the N65C needle. That did not help. I then shimmed the N65C needle 0.020" and that helped a little, so I lived with it for a while. Then I learned another thing that changed with the rubber mount age of Sportster. Harley Davidson Screaming Eagle air cleaner that used to be one of the best values for improved intake air solutions in years passed has become a piece of junk. Instead of an oiled K&N filter and an aluminum backplate that used to be part of SE air cleaner we now get a crappy dry (non-oiled) filter that is prone to developing holes in the filter medium and a plastic backplate that warps. Price has not gone down, however. Once I noticed holes in my 3 month old SE filter, I went back to the dealership I bought it from and was told that the filter was out of the 90 day warranty, however they would, in good faith, replace the filter for me for free. I thought that was nice of them and at that point chalked the filter failure to statistical probability. A few months later the new filter succumbed to the same failure. This is when the after market vendors began to catch up to the rubber mount bikes and a few options other than SE became available. One of the options was Arlen Ness Big Sucker air cleaner. It cost less than the SE unit, but came without a cover, the cost of which brought the total cost to about the same as the SE. The Arlen Ness Big Sucker is a very high quality product, much more so than the SE. The backplate is cast aluminum and has built in breather ports. The filter itself is an oiled type filter just like K&N, though Arlen Ness insists it is NOT a K&N and was designed specially for them. After installing the Big Sucker I realized that on top of being a poor quality air filter, Screaming Eagle filter also was pretty restrictive. After changing from SE to Big Sucker it became clear that the bike ran way too lean as it was jetted for the SE. I had to jet up to 175 main, 45 pilot and keep that shim under the needle. The bike performed great! Finally it had good power and good driveability with no carb pops and smooth acceleration. I later went back to the stock needle from the N65C and also shimmed it 0.020". This change produced a gain in top speed performance. It seems that the needle for rubber mount Sportsters was yet another component that changed and for the better. This stock needle seems like a great fit for the bike once shimmed. This was the last time I adjusted the carburetor, besides adjusting the idle mixture and idle speed to correspond to the new tune. Some time later I decided to try and drill out my stock mufflers as many people have done, so I drilled a 1/2" hole in the baffle plate of each muffler and installed them on the bike instead of HD scripted ones. Another surprise. That stock exhaust with a drilled out baffle plate made the bike run better than the $200 pair of "performance" mufflers. Another HD gimmick, just like the SE air cleaner. So, I've been running the following setup since summer 2005:
Arlen Ness Big Sucker air cleaner
Stock exhaust with 1/2" holes in the baffle plates (almost as quiet as stock)
175 main jet (stock on my year and model Sportster)
45 pilot jet
Sometime in 2007 I switched the pilot jet to 44 and the bike runs just as well, so I have stayed with 44 pilot jet. The rest if the carb adjustment did not change.
Idle mixture and idle speed adjusted as per instructions outlined here.
I used my "seat of pants" to tune the bike, but recently (after about 20K miles) I had a chance to put it on a dyno and was pleasantly surprised to see that the bike was tuned very well. Not maxed out on the power and torque, but air to fuel ratio curve was very nice and smooth. Here are the graphs from the dyno.
