First and foremost, fuel injection on four-stroke motocross bikes is a fait accompli. No one is fighting it. It’s a done deal. The Yamaha YZ250F was the last holdout and they quit holding out this year. There is no going back to the venerable carburetor in motocross—even NASCAR was forced to bite the bullet in 2012. But, has it really changed anything about NASCAR or motocross?
Throttle-body injection, as used on modern motocross bikes since the release of the 2008 Suzuki RM-Z450, was introduced to aircraft engines in the 1940s and to automobiles in the 1980s. Throttle body injection simply replaces the carburetor in its old location with fuel injetcion. The induction mixture passes through the same intake tracts as before. The justification for throttle body injection as opposed to direct injection or multi-point injection is its low cost. Many of the carburetor's supporting components can be reused, such as the air cleaner, intake manifold, head and fuel line routing. This postponed the redesign and tooling costs of these components to make the carb of EFI swap as cheap as possible.
There is no doubt that fuel injection is more high-tech than carburetor induction, but is it better? Here are both sides—you be the judge.
JETTING: A fuel-injection system atomizes the fuel by forcing it through 12 small nozzles at high pressure. A computer reads sensors that report the throttle position, intake air pressure, water temperature and other details to determine the precise amount of fuel needed.
ALTITUDE COMPENSATION: Complete combustion can only occur if the air and fuel are present in the exact stoichiometric ratio. A fuel-injection system’s oxygen sensors adjust the air-to-fuel ratio in real-time. Thus, a fuel-injected bike can perform as well at 7000 feet as at sea level.
COMPLEXITY: A fuel-injection system is much like a maintenance-free battery or any household electronic device. It will work without any maintenance until it doesn’t work. But, because of its computer, sensors, fuel pump, high-pressure fuel lines and wiring it has a myriad of potential trouble zones.
ADJUSTING: There is no need to adjust anything because all adjustments are handled by the computer. Riders can reprogram the fuel maps with readily available software when the need arises.
WEIGHT: Because of the need for a 12-magnet magneto system, fuel pump, thicker gas tank, sensors, larger ECU and throttle body, the average fuel-injected bike gains close to five pounds.
POWER: One look at a dyno chart will reveal that a fuel-injected bike makes two to three more horsepower at 5000 to 7000 rpm. In addition, the throttle response is improved across the board.
FEEL: There has never been a carbureted bike that had the crisp, responsive and instantaneous power delivery of a fuel-injected bike. EFI has totally eliminated the dreaded bog that came with antiquated brass jets.
Keihin FCR carburetor.
JETTING: A carb’s fuel flow is determined by engine vacuum. Brass orifices (called jets) regulate the amount of fuel. A properly adjusted carburetor can deliver the perfect amount of fuel without the need of electronic sensors, computers or pumps.
ALTITUDE COMPENSATION: A carbureted motocross bike must be jetted differently to run at 7000 feet as opposed to sea level. This requires changing jets. Since the vast majority of race tracks are from sea level to 4000 feet—most jetting issues only require a simple fuel screw adjustment.
COMPLEXITY: From a mechanic point of view a carburetor has a large number of parts, but with the exception of the slide, floats and accelerator pump these parts are stationary and not subject to wear. If something goes wrong with a carb, it is a simple task to diagnose the problem and fix it. Conversely, fuel injection troubles can only be diagnosed with an ohm-meter, tech manual and new parts.
ADJUSTING: There is a learning curve required in manipulating a main jet, pilot jet, leak jet and fuel screw. However, once the jetting is set for the local conditions it rarely needs changing.
WEIGHT: The cast Keihin FCR carburetor is light (and apart from a cable, it requires no supporting parts in terms of pumps, maps, thicker gas tanks or magnets).
POWER: Fuel-injected bikes may make more power down low, but from the midrange on up most carbureted bikes are more powerful (although not all). Switching to EFI does not produce more power. For example, the fuel-injected 2009 CRF450 made more power from 5000 to 7400 rpm, but the carbureted 2008 CRF450 made over two horses more from 7500 to 10,000 rpm.
FEEL: There is no doubt that some poorly jetted bikes, like the Honda CRF250, gave carburetors a black eye, but most MXA test riders preferred the organic feel of a carb-equipped bike. They seem to have the ability of produce more power and have that power readily on tap when compared to the over-caffeinated feel of an EFI bike. It is true that Professional Supercross racers need the instantaneous response of EFI, but the average rider does not. Fuel injection is one of those changes that doesn't actually changes anything inherently important (we’re still going around at the same speed, just with a different fuel delivery system)—what it does do is add complexity and cost to already expensive machines.
KTM Motorcycle tests