Increasing horsepower on your Harley increases the value and fun factor.
The Harley-Davidson 1200 Sportster is frequently modified and customized with both Harley-Davidson original parts and aftermarket add-ons and accessories. The 1200 engine can be easily modified to produce more horsepower and torque. Kits are available from Harley-Davidson Screaming Eagle, Revolution Performance and Weisco. All kits involve essentially the same steps, and many bolt to the original cases with no modifications.
1. Establish a game plan and purchase necessary parts. Determine what big-bore kit and heads you want to increase the horsepower on your Sportster. To accommodate these changes, you will also need to increase airflow by changing your air intake and exhaust system. You will need to rejet your carburetor also and use an aftermarket ignition module. These are available from several manufacturers. Also remove the stock rev limiter and ignition map designed by Harley-Davidson.
2. Remove the engine from the frame and remove the old heads, cylinders and pistons. Thoroughly clean all parts and set them aside.
3. Install new pistons, ensuring that piston pin clips are installed properly and pistons are facing the correct direction. (An arrow usually points to the intake side of the piston.) Using a piston ring compressor, install the new cylinders on the pistons, taking care not to damage the new piston rings.
4. Install the new heads. Using a torque wrench, ensure that all fasteners are torqued to proper specifications. Adjust valves to the specifications provided with the kit.
5. Place the engine back in the frame, tightening all fasteners with the torque wrench. Install the new exhaust kit and the ignition module.
6. Rejet the carburetor and install it on the bike. The kit instructions will often tell you what size jets are needed for the carburetor to function properly at your altitude. Install the new air intake kit.
7. Run the bike through the break-in procedure recommended by the kit manufacturer and have it dyno tested. Depending upon the kit you chose, the engine should be in the 100-115 hp range. If not, rejetting the carburetor using dyno analysis and reprogramming the aftermarket ignition module should provide the desired horsepower.