Induction System Basics - Hot Bike Tech

As a general rule, the S&S Super E and Mikuni 42mm carbs are popular for stock displacement Evo and Twin Cam engines up to approximately 95ci, while the S&S Super G and Mikuni 45mm are best for roughly 95ci and larger engines. Super G carbs can be bored out for additional airflow and various main discharge tubes are available to suit an engine's airflow demands. The Mikuni 48mm Flat slide is best suited for engines in the 107ci to 113ci range and larger. The S&S Super "D" carb is available in two venturi sizes and is sometimes bored out for increased flow. Since the D is designed as a racing carb and does not include an accelerator pump, it is mostly used on large-displacement, high-revving race engines, although some Ds are seen on 113ci to 120ci and larger hot street engines. The Harley-Davidson Screamin' Eagle 51mm CV carb works well with 120ci and larger engines and high-revving smaller engines. As an alternative to using one large carburetor, some engine builders prefer two smaller size carbs for better throttle response and increased total airflow. However, an efficient intake manifold is important for realizing performance gains.

Some aftermarket carburetors include a performance intake manifold while others use a stock manifold. In some cases, a performance manifold can improve performance, but there is no guarantee. Dyno or track testing is the only way to know for sure. I recommend consulting with a knowledgeable engine builder for the optimum combination. On a related topic, when having your cylinder heads ported, always include the intake manifold so it is matched to the intake port and optimized for flow. Moreover, keep in mind that a manifold with the largest plenum volume is not necessarily the best performer, but the manifold should provide a smooth transition from the carb to the port without any abrupt drop-off.

Tuning Considerations
The stock 40mm Keihin CV carburetor is excellent for mild hop-ups on 80 to 88ci engines and even some 95ci engines as long as the carb is rejetted and tuned properly. Rejetting the CV carb will eliminate any low and midrange staggering and coughing while allowing the engine to develop to its maximum power potential.

There are a couple methods for rejetting the Keihin CV carb. Some engine builders prefer a complete jetting kit while others prefer their own mix of individual parts. Certain jet kits are too rich for optimum performance and fuel efficiency. As an alternative to a complete kit, you can replace the CV's needle jet and possibly the slow-speed jet, then adjust the idle mixture jet and you're good to go. If you want to go a step further, you can have a qualified shop modify the throttle bore for greater airflow. The 40mm Keihin CV carb gives excellent low speed response and, if set up properly, provides decent power and excellent fuel economy on pump gas.

For optimized power, airflow through the induction system should be as laminar as possible. Keeping airflow as straight as possible while minimizing intake tract turns will minimize turbulence and restrictions while optimizing flow and velocity. To this end, use a large high-flowing air filter element, which will help straighten the airflow into the carb's throat. Additionally, some air cleaner backing plates include a radiused inlet to reduce turbulence for improved airflow.

When testing intake manifolds, the most important tuning elements are tract length, tract taper, and plenum volume. Larger volume manifolds may perform better at higher rpm but usually provide slower throttle response. When experimenting with different manifold volumes, start testing using the manifold having the smallest volume and increase volume until no performance gain is realized.