2012 Kawasaki KX250 dual fuel injectors explained

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While the KX450 might be a little retarded, the 2012 Kawasaki KX250F has gotten a serious engine management upgrade. Until just a few years ago, MX bikes all used carburetors. The Keihin FCR is a complicated piece of highly refined technology that works extremely well and represents the ultimate evolution of carburetor technology. The first MX fuel injection systems were a little bit clunky. They stumbled right off idle, lacked the crisp throttle response of an accelerator pump, stalled occasionally, and were less than easy to start under some conditions.

The dual injector system helps to bring back the accelerator pump performance of a carburetor. Consider this your warning: shit’s about to get technical. For those that haven’t logged time running a dyno and spent hours looking at an FCR trying to out-think the engineers Keihin paid millions of dollars, these things may as well be magic.

Starting out easy: In a carb, there’s a big hole in the middle that air goes through, a fitting where a fuel line attaches and jets inside that always seem to need cleaning.

A little harder: There’s a main jet, a pilot jet, a needle and an accelerator pump. There is also a choke and often a hot start circuit. These systems work together to mix air and fuel in the right proportions and to make sure that fuel is in the form of a nice even mist and not big drops or rivers. Fuel injection can more precisely deliver fuel, but it lacks the all-important accelerator pump.

See, the thing about the accelerator pump is that it squirts a predetermined amount of fuel straight at the intake ports every time you open the throttle and hold it open. As a tuner, that gives you all sorts of flexibility when you’re trying to make a high-strung race motor run right. Anyway, that squirt of fuel lasts anywhere from less than one second to maybe 5 seconds. By changing a leak jet in the circuit you can adjust it to do whatever you’d like.

Regardless, it lasts much longer than one revolution of the motor and at some point fuel will be getting squirted toward closed ports. It also comes out as more of a stream and less of a mist. That fuel hangs around for a while (not a long time when you’re talking about a motor that opens and closes its intake valves 6,750 times a minute). Eventually, it all evaporates and gets sucked into the motor where it’s burned. Mixing air and fuel in this way means you get a nice evaporative cooling effect, a slightly richer mixture, and a ready-to-use air fuel mixture hanging around in the intake tract. All of those things add up to a really crisp response when you open the throttle, especially right off idle.

A fuel injector works differently. It’s got a pressurized fuel line hooked up to it and a couple of wires that control a solenoid. There’s also a little black box with a miniature Jean Luc Picard inside who drinks green tea with his legs crossed, contemplates the universe and decides when to fire the solenoid. Just kidding. I don’t know exactly what goes on inside the computer. Fuel comes spraying out the injector holes when the solenoid fires and the tiny holes see to it that it comes out in a fine mist. Also, note that a carburetor is a mechanical, analogue device and an injector is digital. The carb supplies fuel to air that passes by, whereas the injector fires off short bursts of fuel to air a sensor tells it is probably passing by. A carb also has the advantage of effectively having several different overlapping maps that can be tuned any way they need to be to suit the motor.

Like I said, the Keihin FCR carburetor is a serious piece of tech. Here’s where the 2012 KX250 comes in: Its system uses two injectors. One is placed in the usual spot, just past the throttle plate where it can take advantage of turbulent air. The other is inside the airbox and pointing toward the throttle body. The concept isn’t all that new — sportsbikes have had dual injectors arranged this way for a long time now. What’s different is that it’s been adapted to work with a single-cylinder motor that’s a lot more finicky and high strung.

Kawasaki’s chart seems to indicate a much harder transition between the downstream and upstream injectors. Without having the bike in front of me to fool around with, it’s hard to tell if they mean that the downstream injector shuts off or if the upstream injector simply provides additional fuel at high RPM and large throttle openings. Either way, it’s still a pretty trick system.

Here’s where things get interesting: The green injector uses the turbulent air rushing past the throttle body at small throttle openings to help mix the air and fuel together. A consistent (homogeneous) mixture burns more completely and that means more power. All of that evaporated gasoline also cools everything it comes in contact with. The cylinder head, valves, piston, cylinder, combustion chamber and throttle body all benefit from that cooling effect. Even the exhaust valves benefit for a moment when they’re open at the same time as the intakes. Cooler parts mean a more dense intake charge which, again, means more power. They also mean stuff is less stressed. That doesn’t necessarily mean that it’s less likely to break, just that Kawasaki can push harder and run an even more aggressive tune and, drumroll…more power. Where does that secondary injector come in? Well, since it sits way back in the air-box, it’s spraying fuel into a much larger and slower moving volume of air. That air is going to take longer to reach the valves and that means more time to get swirled around, evaporated and turned into a nice homogeneous mixture that pulls heat out of anything it touches.

So there you have it. This is real technological progress and I’d say it makes up for the weak launch control they’re selling on the KX450.

Basically, that secondary injector will make the KX250 feel like it’s got an accelerator pump. It creates an air/fuel mixture that hangs around for a minute, better mixing itself up and enables a big dose of fuel to be dumped into the intake when you whack the throttle open. This means that the stumbles that have characterized MX fuel injection are gone, it means when you open the throttle, the bike does what you expect it to, and it means more overall power is available when you want it.


  • T Diver

    Thanks for not making me read the whole thing. Last line – “…it means more overall power is available when you want it. “

    • Sean Smith

      Yup. Harder, better, faster, stronger. Such is the progression of motorcycle technology. Unless you’re talking about cruisers.

      • Robert

        Where do you put the conchos on this thing…

  • Dan

    OK Kawasaki, now stick a pair of these on the Ninja 250, charge me a little more for the upgrade (maybe throw in ABS like the new CBR, whaddya say?), and I’ll buy it tomorrow.

    • aristurtle

      The Euro version has had fuel injection for a couple years. If you want to upgrade a carbureted model, there’s a company that sells a $500 upgrade kit. Cash is tight for me right now but I’m thinking of picking it up eventually, mostly just for the hell of it.

      edit: here’s the link: http://ecotrons.com/Kawasaki_Ninja_250cc_EFI_kit.html
      I don’t think it would provide a significant benefit to performance or fuel economy but it might be a fun project anyway.

      • http://www.thisblueheaven.com Mark D

        Cool! It might not aid fuel economy or power, but it would help emissions, as well as cleaning those tiny motherfucking carbs if you don’t run the bike for 10 days, or having to warm it up for 10 minutes on dry 65 degree day.

        • aristurtle

          It would be nice. I’m just worried that if I start doing this, then I’ll think “well, now it would be easy to put a new exhaust and intake on” and then that would lead to “well, we did all these powertrain mods but the thing could use a suspension upgrade” and then before I know it there’s $4000 in parts on a bike I got for $1000.

  • dux

    Cool. But can they make a light weight dual sport that doesn’t need the top-end rebuilt every 1000 miles?

    • eric

      and doesn’t cost as much as a used superbike…

    • Sean Smith

      Those things are expensive as hell to build and usually have more tech in the motor than a superbike. The piston in a KTM 450 is easily as nice a piston out of a MotoGP bike. A pricey motor is no good without pricey suspension, and most people want a fancy lightweight frame that’s labor intensive to build. Cap it off with lightweight parts, nice hardware and a titanium pipe, and you’ve got light-weight dual sport that’s 98% race bike.

      Your best bet if you want unobtainium like that is to go buy an old XR400, have Thumper Racing build you a 440 motor with a cam, and bolt on some trick suspension. I’ve torn down 440 kits with 300+ hours on them, and all they needed were rings. They make pretty solid power too.

      • dux

        Sweet. May give it a shot. I’m an old DR350 guy so I’m accustomed to overboring.

  • jeremy

    so basically the second injector is the equivalent of putting a squirrel with a can of starter fluid in your airbox?

    • dux

      Where can you get one of those? I wouldn’t mind the extra HP.

      ’87 CBR600

  • http://www.thisblueheaven.com Mark D

    Dude, Picard drinking green tea? Come on. Earl Grey. Hot.


    Cool tech article! Not too dumbed down, not too crazy esoteric.

    • Devin

      You beat me to it.

    • Scott-jay

      Sean can tell a story! Analog – digital descriptions shine.

  • Trev

    I wonder, when are we going to get a “flat-slide throttle body”? It probably is unneeded, but I think that would be some cool technology coming from carburetors to throttle body.

    • Sean Smith

      While I was writing this, I was wondering the same thing. People have done it before, but I think that it’s just not necessary. The flatslide makes a huge difference on a carb because it forces fast moving air past the jets at low throttle openings. That extra strong signal means more accurate fueling and super crisp response.

      With injection though, there’s no mechanical vaccum signal; instead the computer is running off throttle position, revs, coolant temp and ambient air temp. That focused fast moving air no longer matters as much. That’s why we’ve seen size jump from 39mm or 41mm on the FCR, up to 50mm or larger on FI throttle bodies. They don’t need a vacuum signal to operate, so there’s no need to speed up flow as much.

      • Trev

        Good points that you raise. I was thinking that the “unobstructed” air flow, but I agree that it would be a waste seeing as it doesn’t get it’s information from any the mechanical vacuum signal.

  • Pete

    Arg. Too much tech this early…

    Great explanation on the carb side. I’ve always known what accel pumps do but when I try to explain it I can never do so in less than five minutes. Of course the last time I had to do that was when I was explaining to my wife why I needed one…

  • KeithTurk

    I enjoyed the read… it’s simply done well… not overly technical until you realize you just learned the purpose and manner in which an Accelerator pump and second upstream injector work

    That makes it Just right…


  • Dan

    “A carb also has the advantage of effectively having several different overlapping maps that can be tuned any way they need to be to suit the motor.” — I think you mean a fuel injection system, not a carb here.

    • Sean Smith

      Nope. I’m explaining how a carb works in language that most people are familiar with these days.

      If you drew a chart that had lines for the pilot, main, accelerator pump and needle, they would overlap. On a Keihin FCR, you can tune all of those circuits individually and really get a good tune. The flat-slide helps too, as well as the shape of the thing and the choke and hot-start circuits. There are even more things designed into it that have an effect on tuning, but the point of them is to make it so that when you change a jet, that you get predictable, repeatable results.