Layla and I met after work to talk about her new Sportster. “What about performance?” she asked. I sat back and considered that this 1200 Custom Sportster model comes all dialed in with plenty of power from its completely redesigned Evolution 1200 power train. With performance cams and a 9.7:1 compression ratio, it had more torque than its predecessor. Redline on the 1200C is 6,000 rpm, with a power increase of over 15 percent compared to the 2003 base domestic 1200 engine. The combustion chambers and ports are high-flow designs, resulting in an increase in potential airflow. I keyed on the word “potential.” Since the engine has performance potential, but is limited under the restrictions of the EPA and legislative guidelines, two elements could be improved upon: intake and exhaust breathing.

“Let’s change the carb and pipes,” I said. “We’ll ride over to my place and scour the 2004 H-D catalog.” Harley’s Parts and Accessories division distributes a line of Mikuni carburetors through the Screamin’ Eagle line. I’ve had nothing but good experience with Mikuni, so we ordered the 42mm flatslide carb, the Screamin’ Eagle high-flow air cleaner kit, a required adapter kit and a tuner kit in case jets needed adjusting.

Then we searched for pipes and found limited Screamin’ Eagle options, so we reached outside the factory and considered several of the companies that build performance exhausts for the new 2004 Sportsters, including Vance & Hines and the new Samson Caliber.

Layla took one look at the Caliber two-into-one unit with complete protective heat shields and our decision was cemented, the new tech project was underway. We ordered the parts, and when UPS arrived with the goodies it was time to go to work.

Sometimes the H-D instructions can be confusing, and the Mikuni came with very complete, yet complicated, directions. I read all the instructions first and sorted through the material that didn’t apply to this model. It’s easy to slip onto the wrong page; this Mikuni kit is designed for use on 1995-and- later XL 1200s and all twin-cylinder carbureted Buells.

Confidence high, I tore into the bike like a madman, removing the stock air cleaner, the carburetor and the exhaust system. Layla carefully organized every nut and bolt in case we wanted to reuse a fastener during the installation or return the bike to stock. We carefully noted the position (for future reference) of the vent/vacuum hose line on the back of the carb (it runs to the fuel petcock), the fuel line and the overflow line off the float bowl.

Of course we turned off the gas and disconnected the battery before we messed with the bike. As it turned out, we were forced to remove the gas tank, so I popped the seat off first.

Our next move in preparation for the Mikuni carb operation called for unplugging a number of wiring connectors under the tank. There’s a large plastic backbone clip that needed to be removed for rerouting the stock throttle cables. We disconnected all the connectors, removed the bulky clip, reconnected them and tie-wrapped them into place with the three supplied tie-wraps.

While Layla disappeared to make sandwiches I read over the instructions regarding the choke. I was confused. The Mikuni uses the choke knob and mechanism from the stock CV carb, sorta. I unscrewed the black plastic screw on the back of the CV and removed the cable, spring and plunger. The instructions called for removing and discarding the black plastic nut on the carb, then using the spring and brass plunger from the Mikuni kit. I checked the kit—no plunger or spring— then I removed the black plastic nut, and the spring shot across the garage floor. I scrambled after it. Did I miss anything? As it turned out the plunger remained in the carburetor body. I dug it out, then the picture cleared. I switched springs and plungers and replaced the choke unit in the back of the carb.

Next, we checked the condition of the rubber intake manifold seal. Since it was clean we smeared it with liquid dish soap and slid the new Mikuni into place. The air cleaner backing plate holds the carburetor firmly in place.

We then rerouted the throttle cables from the right side across the front of the frame neck, down the left side of the bike and in toward the carb behind the front rocker box.

Here’s where the rewiring maneuver became critically obvious. We carefully adjusted the pull throttle cable first, with just 1/8-inch of slack, then the push cable so it didn’t bind with the pull cable.

We made absolutely sure the cables were lubricated and wouldn’t bind when the front end was turned from lock to lock. Again we studied the three different backing plate configurations for 2004 XLs, earlier Sportsters and Buells.

We installed the 0-ring fitted adapter, gasket and SE backing plate with the supplied screws. Then the breather fasteners that hold the backing plate in position and double as breather tubes were screwed into position and two new breather hoses were positioned. Here’s the twist to the generally simple configuration. The illustration with the instruction indicates a gasket between the backing plate and the adapter, but it says, “Gasket shown for illustration purposes only (not included).” But was it included with the SE High Flow Kit? After a while I go blind from reading instructions mixed with part numbers. We installed the air-cleaner element with dabs of fastener Loctite.

That encompassed the first element of the new high-flow air cleaner kit. This kit is designed for only 2004 models and required an additional enhanced flow bracket with the Mikuni installation. The bracket was a no-brainer, yet confusing. It replaced the molded form that fit behind the air-cleaner cover and opened the area up considerably.

A bracket fitted to the air-cleaner element that lifts the font of the air cleaner a ¼-inch out from the element more than the rear, then two spacers were added and the final countersunk Allen screw spun into place with Loctite.

We moved on to the Samson Caliber exhaust assembly. First, we needed the exhaust manifold rings and snap rings from the stock pipes. There’s a simple Sears Craftsman spreader tool (9-4662) that makes this job a snap the first time I used it. Without it I’d burn through a half hour struggling with those springy exhaust bastards. With this tool the spreading operation took seconds without breaking a sweat.

The operation was simple. The system comes with complete heat shields (including the muffler), a strong pipe bracket, two high-tech baffles and all the hardware and clamps. The baffles supplied are a Samson patented two- step design to improve exhaust velocity (which increases torque). The perforated cone portion also helps capture sound better, reduce high frequency noise, and results in a rich throaty sound. Two baffles are supplied, a 1 3/4 and a 2-inch.

We installed the muffler bracket with Loctite under the front pulley shroud. It slipped right into place.

We needed to remove the rear brake linkage, and then install the pipe system loosely. We left it that way until all the heat shields were attached.

Layla made sure, as I slipped each hose clamp into place, that it was facing a chosen direction. We discovered that if we turned the nose of each clamp-end down just a few degrees with needle nose pliers, they fed into the clamp housing more readily. With all the shields in place we tested the fit with the system bracket and checked clearances from the brake linkage. We tightened the brake linkage, and then tightened the Samson system at the head. Finally we tightened the muffler bracket bolt.

We were drawing this assembly to a close. I chose the smaller baffle for enhanced torque and slipped it into place after some grinding on the cone end for easy installation. (According to our sources, the larger diameter baffle is designed for big-inch engines and/or bikes with lots of head work.)

We carefully installed it to align properly with the ¼-inch mounting hole and installed its Allen fastener. That left only one last item, the chromed billet aluminum Caliber muffler cone. A dab of Loctite, and the job was done.

For further information see your HarleyDavidson dealer

The Screamin’ Eagle parts involved are: 29042-04 - SE high-flow air cleaner kit
29848-04 - adapter kit
27454- 01 – SE flatslide carburetor kit
29564-98 - Race Tuners kit.