Harley-Davidson powered their 1985 Sportster XLH with a 997 cc engine that produced 50 horsepower at 6,000 rpm giving it a power-to-weight ratio of 5.98 pounds per horsepower, with the stock components. You can, without serious tooling or mechanical experience, make some changes to increase horsepower and torque. Other changes require significant modifications to the engine.
The most popular starting point for boosting performance is known as a "Stage One" upgrade through Screamin' Eagle, otherwise known as "paying your Harley tax." This includes replacing the stock air cleaner element with a low-resistance air cleaner, removing the stock exhaust and installing a low-restriction exhaust, and replacing the jets in the carburetor with larger ones. It is recommended that you use a low-restriction performance muffler, and avoid using straight-, or drag-pipes, because they have no backpressure and will make it difficult to tune the exhaust. Anti-reversion cones -- also called powercones -- help provide some backpressure and prevent exhaust-gas reversion. Making these changes effectively opens up the engine and allows it to breathe as the factory intended, increasing horsepower and throttle response.
Upgrading the ignition coil to a high-output coil, along with performance spark plug wires, will give a small horsepower boost along with a slight mileage increase by creating better flame-front development and reducing wasted free hydrocarbons exiting the exhaust. Screamin' Eagle offers a performance electronic ignition module that will allow you to tune the power curve and adjust the rev-limiter to maximize performance according to your riding style. Performance points are recommended if you have converted to a points ignition. Use high-performance points with a built in self-lubricator for good ignition performance with less frequent need for points adjustments.
Increasing the swept volume of the engine by boring it to 1100 cc and installing high-compression pistons will increase your horsepower and torque significantly. Mid-grade or premium gasoline will be necessary to prevent preignition or detonation once engine compression exceeds 9.5-to-1. Performance cams are available from Screamin' Eagle and other aftermarket sources, that will enhance your power curve and increase your torque band significantly. Port and polish the intake passages in the heads, and use a five- or seven-face grind on the intake seats.
Tampering with the intake and exhaust, or making any changes to the emissions on the bike, can be a violation of state and local laws. Check for ordinances against sound emissions and exhaust-gas emissions before making modifications to a bike that is intended for road use. Do not take half-measures when modifying the intake and exhaust systems. Changing the pipes or intake without increasing the size of the jets will lead to a dangerous lean-running condition that will cause the engine to run hot and can cause piston damage. Give particular attention to maintenance on other areas of the bike when increasing performance. The performance of the brakes and the condition of the tires become infinitely more important as you increase your capabilities and use these capabilities during maneuvers in traffic.