FCR Carb Explanation

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jjs777
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FCR Carb Explanation

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The below info was quoted by one of the users. And consequently the explanation was blessed by someone that knows. No not me. But thanks any hoot.<br><br><br>ok here goes!<br><br>Instead of a throttle butterfly, we use a slide mechanism in the middle of the venturi. It opens and closes off the supply of air to the engine. But now, because we are varying the size of the venturi, we have to also vary the size of the fuel jet. No problem, as we’ll just add a tapered needle to the bottom of our slide that runs down into the jet at partial throttle openings. Once we open the slide all the way, we are back to a standard venturi size, the needle is pulled out of the jet and we’re back to our fixed air to fuel ratio. At least we are if the jet is the right size!<br><br><br><br>This type of carburetor is called a direct slide or simply a slide carburetor. They are still the carburetors of choice for racing motorcycles. The reasons are they are easy to tune, only have one major moving part, and when the slide is wide open, they don’t present any restrictions to the airflow. The famous "flat-slide" carburetors are built this way. The slide itself is somewhat flat if looked at from the side when compared to the older round slide carburetor. The major benefits are lighter slides, less potential slide friction, and the ability to build a shorter venturi passage, increasing potential airflow a bit more. They are also harder (read more expensive!) to build.<br><br><br><br>The downfall of a slide carburetor is they don’t work as well on the street. This is because they don’t work well over a wide range of throttle setting, engine speeds, and engine loads. For example, let’s say we’re riding along in a higher gear at fairly low engine speeds. We want to accelerate quickly so we yank open the throttle on our slide carbs but nothing happens. Or least that is what it seems like at first. What we’ve really done is opened up the air passage into the cylinder all the way, but the engine’s piston is still moving slowly. The engine gets a big gulp of air at first, not enough fuel to go with it and it has just stalled out momentarily. We can help this if we also squirt a little extra fuel down the throat along with the initial gulp of air, but our engine is still turning slow. Plus, we now have the added complication of that extra fuel squirter, commonly called an accelerator pump. If we’re not careful in using the throttle, we’ll also be squirting extra fuel in there all the time, certainly not helping our gas mileage out.<br><br><br><br>Because the engine speed is still slow, the carburetor size is not optimal, and is in fact, too big. It will eventually catch up, but our acceleration won’t be as good. This is why we don’t want to just grab a big handful of throttle on a slide carb engine, at least at low engine speeds. But all you old timers remember that now, don’t you! Still, we like this being able to vary the size of the carburetor to the engine’s speed and load so…<br><br><br><br>Yep, you guessed it. We’re now talking about constant velocity carburetors. To do this trick, we simply take control of the slide away from the rider. We still need a throttle so we’ll give ‘em a throttle plate valve back to control the overall airflow. We control the opening of the slide by balancing it against the airflow. We can sense the airflow by porting the pressure from various places in the carburetor itself. As the airflow increases, it’s pressure through the venturi decreases, relative to outside pressure, and this allows the slide mechanism to open. Good old atmospheric pressure back on the job!<br><br><br><br>Now when our rider yanks the throttle open, they only open the throttle plate. The slide opens up only as airflow increases, creating an ever-bigger carburetor. The velocity of the air through the venturi remains basically constant, hence the name. When we have less volume of air to flow, we want to keep the velocity high to ensure a good pressure drop across the main jet, good fuel atomization, optimal carburetion sizing, etc. To do this, we have to have a smaller opening. Then as we have more airflow, the carburetor throat can now be larger. So we now move more air but at the same speed or velocity that creates a good pressure drop, and all the rest of those good carb things.<br><br>Grianaig (AKA RabC)<br> <p>-John<br>95 FZR1000<br>90 FZR1000<br>90 FZR1040</p><i></i>

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