GUNNING FOR THE LEAD  Part 2                                      Printer-friendly format
On-Track Tuning with the "carb trigger"

By John Copeland

A month or so ago we took a close look at the Kwik-Set Carb Adjuster (commonly called the carb trigger) and how it's used to adjust the Walbro WB3A carb. This little gadget has become standard equipment on virtually every front-running Yamaha powered kart in enduro racing. And more and more sprinters and dirt racers are finding the carb adjuster to be another weapon in their arsenal too. Most racers know that they can use the adjuster to lean down the fuel mixture on the straightaway, and that that's a desirable thing to do. But to really get the most out of the tuning that the trigger can allow you to do, there's a lot more to it than just "pull it on the straight, let it go in the corners."

The carb adjuster can't make you faster all by itself. Obviously the driver has to move the handle in and out, but there is a lot more to using it to the max. First of all, you have to understand what the engine "wants", and when it wants it. You have to be able to anticipate what the engine's fuel needs are going to be, so you can adjust for them at the required moment. Remember that the fuel/air mixture going into your engine serves more than one function. Of course, it's the source of all the combustion energy that you'll use to drive the kart forward. But it's also a valuable coolant that helps leach heat away from delicate engine parts.

As an energy source, it's important to remember that all the horsepower your engine is ever going to produce is stored in the fuel. The more fuel/air mixture your engine can burn efficiently, the more power it will produce. The tricky part is, your engine's fuel requirements are not constant. And even worse, they're not even consistent. As RPMs go up, fuel demand goes up, for a while. Then, as RPMs continue to increase, fuel demand begins to drop. Fuel demand is a very complicated subject, and the theoretical details are really not important to us, in a practical sense. Suffice it to say, all other factors being equal, the engine's optimal fuel demand roughly parallels the torque curve. At low RPMs the engine can only efficiently combust so much fuel and air mixture. As the RPMs increase and the engine gets into it's most efficient and powerful combustion range, it demands more fuel to maintain that combustion. This is the area of highest torque output. That torque output is governed by many factors, including the port configuration, compression ratio, ignition timing, and exhaust system configuration. But, in general, when torque is highest, fuel demand is also. As RPMs continue to go up, torque begins to drop off and the engine does not require as much fuel to maintain combustion. That decline in fuel demand continues throughout the upper RPM range.

With that information, it's pretty easy to understand why the carb adjuster is such an advantage in maximizing performance. Unless you can lean down the mixture at higher RPMs, the engine will just be too rich to run at it's best. But what about the fuel's second important job, as a coolant. Here's where understanding what your engine's needs are can add another dimension to what you can do with the carb adjuster. Fuel manufacturers know how important the heat absorption characteristics of their fuels are. You see, when the carb sprays fuel into the air stream in the carburetor throat, only a portion of that fuel is vaporized. The rest is atomized.

Hey, vaporized, atomized, what's the difference? When a liquid evaporates, it changes to a gaseous state. That's vaporization, and it absorbs a significant amount of heat in the process. It's called the "heat of vaporization" for you chemistry types. Put a drop of fuel on your arm and blow on it. Feel the cold? That's it. But atomized liquid is still liquid, only broken up into tiny droplets. As the temperature around those droplets increases, they too will vaporize, taking some of that heat away in the process. The rate at which a fuel charge vaporizes, and where in the engine that happens, is at the core of the fuel manufacturer's work. As we've said many times before, gasoline is a "vegetable soup" of chemicals, each with different characteristics, including different boiling points. Gasoline components (they're called "fractions") with lower boiling points vaporize first, some right in the throat of the carburetor. Others vaporize all along the inlet tract, as the temperature reaches their boiling points. And some fractions don't vaporize until they actually touch the hot piston crown. These higher boiling components are invaluable in absorbing heat that would otherwise lead to engine failure, either "sticking" or otherwise.

With that information in hand, the carb adjuster now takes on a great deal more importance. Giving the driver the ability to pour additional cooling into the engine at a critical moment is not only a means to avoid engine failure, it is an important performance enhancement.

Piston ported engines like the Yamaha are extremely sensitive to temperature. We all know that an engine that gets too hot will stick, but, even before it reaches the sticking point, performance will begin to deteriorate. Experienced racers can feel this and richen things up, but too many less experienced drivers never get the warning and, Opps!, it's stuck. Remember how we said that the engine's fuel demand roughly corresponded to it's torque output? Well, the heat output also follows the torque curve. Higher torque means higher heat. With the carb adjuster, you can anticipate this and help keep your engine in it's "comfort zone" heat wise, even through this high heat range.

We all know the feeling of sticking an engine on the "pull" coming off a corner. It happens because the engine is in it's peak torque range, producing maximum heat, and the core temperature of the castings was just too high to tolerate the additional load. Bingo! You've exceeded the limits that your clearances will allow and your race is over. But what if you could dump a big mouthful of cooling fuel in there just before you called for all that torque? Closing the throttle helps some. But closing the throttle shuts off the air and most of the fuel. No, a better answer is to open the hi-speed needle a bunch, just before you lift for the corner. If you time it right, you'll never even feel the engine go rich before you lift, but the big gulp of fuel will drop the internal temperature of your engine a lot. The carb trigger is ideal for this. It opens the needle more than enough and quickly, and without taking your hands off the wheel at a time when you probably need to be holding on (white knuckles?). If the carb adjuster is set correctly, you'll probably need to lean it back down a bit just as soon as you're back hard in the throttle. If it blubbers off the turn, a quick squeeze will clean things out right now! But, most importantly, you've sucked some potentially damaging heat away and you're ready to pull hard off the corner.

Now let's take an imaginary lap around the track and see how the carb adjuster can help us. Off the last turn onto the front straight we're already pretty fast, and the carb adjuster is pulled part way to keep the mixture "happy". As RPMs go up on the straight, we ease the adjuster in, feeling how the engine responds. Pull too slow and the engine begins to richen up and get too fat. Pull too fast and the acceleration sags down. Just keep feeling for it. The engine will tell you what it wants. At the end of the front straight we've got a pretty long fast sweeper. It's not flat out, but it's pretty fast. As the chassis gets loaded here, the engine always loses RPMs. Making the engine work harder, and pulling down into lower RPMs means more torque, and more heat. We'll feed the adjuster out some here, feeling again for what makes the engine pull the best. As RPMs increase off the exit, we'll pull it back in leaner again. We've got just a little short chute, then hard braking for a tight corner. We'll be back down on the clutch here, and the engine always gets hot by the time we're off the corner. Just before the spot to lift, dump the adjuster all the way out. We'll make sure we suck some extra heat away before the hard pull off the corner, and this way, when we hit the loud pedal, the engine will have something to burn. In the instant between when we let the adjuster out and when we lift and brake, the engine goes very rich, but it happens so quickly, we never feel it. As quickly as we dumped the handle, we're pulling it back in to help the engine pull clean through the corner. All the time we're feeling for that "sweet spot" in the mixture; out a little, back in a touch. As the clutch hooks up and the engine begins to gain RPMs, we gradually pull it all the way in down the next straight. If we've accidentally set the adjuster to make the mixture too lean when it's pulled all the way, we just need to let it back out some if we feel it start to nose over on the chute. If the temperature starts to get higher than we'd like, we can quickly richen it up to drop the temp, then reach over to richen up the low end needle and pull the adjuster back in.

The key here is to anticipate what the heat loading and fuel demands of the engine are going to be, then to give the engine what it needs to give best performance. The mixture part you can do more or less "seat of the pants" while it's happening. But to get the most out of the temperature control aspect of the adjuster, you have to think ahead of yourself. Watch the temp gauge and understand the pattern of heating up and cooling down that repeats lap after lap. Armed with that knowledge, you can help the engine to perform at a higher level. It takes practice, and it takes the ability to think ahead of where you are on the racetrack at this very instant. But it's what separates the front-runners from the also-rans. Now get out there and feed that need for speed. See you next month.


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