Induction System Basics - Hot Bike Tech

In the last Motor Series installment, we finished a two-part series on the valvetrain, concluding with pushrods, rocker arms, and the valve spring package. This month, we'll turn our attention to the induction system. In the good old days, induction systems were rather simple: one or two Linkert carbs, maybe a polished velocity stack, maybe not, and we were good to go. Maybe that wasn't the best combination for performance, but at the time, we thought it was. Today, induction principles are the same but more complex if you have electronic fuel injection (EFI). As you probably know, the Factory switched over to EFI for all 2008 models. Like it or not, just as Evos have taken a backseat to Twin Cams, the venerable carburetor is subtly being relegated to backseat status. Nevertheless, carburetion has its place and will persevere. You dance with the one who brought you, so we will talk a bit about both EFI and carburetion.

Regardless of whether your engine is fuel-injected or carbureted, the important principles remain the same. If you think of your engine as an air pump, performance gains can be realized by moving more air through the engine. The induction system refers to the system of components that move air and fuel into the cylinders. The system begins with the air cleaner and ends at the intake valve in the port. Sandwiched between the air cleaner and intake valve are various components, depending on whether the system is carbureted or fuel injected. The induction system's passageway for moving air and fuel is called the intake tract.

With a carbureted engine, the induction system consists of the air cleaner, carburetor, intake manifold, and cylinder heads. The carburetor mechanically manages both air and fuel flow. In contrast, an electronic fuel injection system includes an air cleaner, throttle body (TB) for controlling airflow, fuel injectors for metering fuel, intake manifold, processor called an Electronic Control Module (ECM), and several sensors that feed information to the ECM. The ECM calculates the correct amount of fuel required for a given engine rpm and engine load, and determines the precise moment at which the fuel injectors release the fuel into the intake tract. Fuel injectors are typically located in either the intake manifold or intake port, although some are fitted into the cylinder head to spray fuel directly into the combustion chamber.

In stock trim, the Big Twin engine is severely airflow limited. Increasing airflow through the engine will increase power as long as the quality of the air/fuel mixture and combustion process are not degraded. The air cleaner and exhaust system are typical starting points for increasing airflow, because both are highly restrictive and easy-to-replace bolt-on parts. Additionally, the late-model Keihin 40mm constant velocity (CV) carburetor is another bolt-on induction part easily replaced, but it is not a major airflow bottleneck until engine displacement is increased. Other major components that increase engine airflow are the camshaft(s), cylinder heads, and larger displacement through boring, stroking, or a combination of both. However, these parts require internal engine modifications, and only the cylinder heads are considered part of the induction system. By the way, we discussed cylinder head science in the Vol. 39, No. 8 issue of HOT BIKE.

Here's my buddy's dual-carb 98ci kick-start Panhead, circa '66. Then, times were simple and carburetion was clean-cut and unencumbered. A single bored-out carb or dual-carb setup did the trick. Foolishly, we didn't worry about wearing out cylinders and rings by running air cleanerless. And to set history straight, we didn't call them bobbers; they were choppers with a bobbed fender.	Here's my 74ci XLCH, circa '68, with bored-out Tillotson carb and velocity stack. Like my buddy's dual-carb Pan, carburetion in the '60s was clean and neat. Kick-start engines were "choked" by either installing a tickler in the float bowl or covering the open stack with your hand while kick-starting. The Tillotson was replaced soon after this photo was taken with the newly introduced S&S carb.	Stock V-Twin air cleaner assemblies are restrictive. Here's a high-flow K&N-style gauze and wire mesh a/c element that's washable and re-chargeable. Also seen is a top-end breather kit that runs separate vent hoses from each head into the atmosphere.
S&S Super carbs come with a nostalgic teardrop air cleaner housing. Be sure to install a high-flow air cleaner element.