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| Features |
| Tech Talk: Road Racing the Yamaha KT-100 |
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by Mark Clevenger, C-C & Sons Racing
Mark Clevenger is a key member of C-C & Sons Racing. As a three-time IKF Grand National Champion, Mark regularly participates with SWRA and provides trackside support at various races on a contract basis. Your input on this article and/or future installments is welcome. Feel free contact Mark Clevenger at clevengers02@juno.com
So many racers dream of finding the illusive “set-up” or “package.” Is it black magic? I’d rather say it’s a just combination of ordinary elements that work together. A fast KT100 combination is like a finely tuned musical instrument. All the elements are tuned such that they work together in unison. A completely tuned package consists of:
1. Optimal Gearing
2. Proper Clutch Set-up
3. Carburetion Settings
4. Pipe and Flex combination
When these elements are all working together, your engine will not only run like a speeding bullet; it will run much more reliable.
Gearing
Gearing is probably the single most important element to the combination. Changes of 1-2 teeth on the rear sprocket can make .5 – 2.0-second differences in lap times. The gear ratio provides the kinematic relationship between the engine and drive axle. That is, it relates how fast the drive wheels turn for a given engine RPM.
When making gear changes it is extremely important to note changes in wind, drafting, and tire circumference. Likewise, you may need to adjust your gearing if any of these parameters change suddenly. It is best to analyze gear changes using MPH and lap time. Since you are changing the kinematics of the drive train, you cannot assume you are going faster if the RPMs increase. Of course, this is only true if a gear change has not been made.
This is a great place to put your high tech data acquisition unit to work. Histograms are great for analyzing how much time is spent at different MPH and RPM ranges. Obviously you want to spend more time at high speeds than slower speeds. Use the histogram to make sure most of your time is spent in the RPM range your engine makes power. Fast laps tend to generate bell curve shapes as in Figure 1.
 | | Figure 1: RPM Histogram Plot. Note Profile Somewhat Resembles Power Curve. |
Clutch
The clutch is an often overlooked and an even more often misunderstood key to a fast KT100. Essentially, the clutch allows the engine to build up torque before applying it to the rear wheels. Today’s karting clutches are speed-biasing centrifugal clutches. That is, they kick in due to centrifugal forces from the engine speed at a given RPM.
The RPM at which the clutch engages is often referred to as the “stall speed.” The term stall speed comes from that when the brake is depressed, the engine is free to rev until the clutch engages. When the clutch engages, the motor either has to overcome the brakes or stall. Likewise, on the track when the clutch kicks in, the RPMs will remain constant for a brief period of time which MPH is increasing. If you are using data acqusition, look for these traces as in Figure 2. You want to minimize the flats in the RPM trace while increasing the attitude in the MPH graph.
 | | Figure 2: MPH and RPM Plotted versus Time. (Notice the flat area in the RPM trace while the MPH is increasing) |
Common thought is that slipping the clutch higher results in greater corner exit speed. Not true. You might have heard that the clutch should be set to engage at peak torque. This too is not always true. The clutch should be adjusted so that it comes in to maximize use of the engine’s torque band. Many Yamahas equipped with the latest pipes have a dip in the torque curve just after peak torque. You do not want the clutch to engage and then the torque to fall off. You need the torque to lug you out of the corners and engage the clutch.
Carburetion
Often times a mistuned carb is confused with an improperly tuned clutch. The WB3A carb on the KT100 is a sensitive beast and can be difficult … sometimes impossible to tune. Accordingly, carb tuning is what separates the good tuners from the great tuners.
Many racers like to adjust the low side needle to get off the corners and the high side for high-speed straights. This strategy works great in sprint applications where gearing helps you race through holes in the power band. Over the years, I have found for road race applications it much more effective to get close with the above strategy. Then, fine-tune the carb using the low side needle only to control temperature (CHT) and the high side needle to make the engine run clean and crisp and maintain the EGT I am looking for.
Generally speaking, EGTs are a sign of power. The more heat out the more power out given the same amount of fuel. Therefore, you want the EGTs to rise as throttle is applied and you work through the power and. In a Yamaha, you should notice the EGTs level off or drop a few degrees towards the end of a long straight. Any significant loss in EGT is a sign of denotation. When denotation occurs, not all the fuel is consumed during combustion. The unspent charge cools the exhaust as the engine signs off and EGT consequently falls.
Besides target EGT and CHT, most experienced tuners read the spark plug. While its effectiveness is continually debated, the fast guys still do it. I find reading the exhaust header a bit more informative. You are looking for a real light tan color on the inside of header or flex. If a white residue is left, you’re too lean. A dark tan or black color means you’re too rich.
The best way to tune the carb is simply by seat of the pants. This is something that takes experience, a good ear, and hopefully only a couple pistons. Most of the fast guys run a carb pull or trigger which adjust the high-speed needle. The idea is to provide ½ - ¼ turn adjustment on the high side. This is especially useful in road race applications as you can use the fuel to help get off the corners, yet lean out the high side once up on the pipe.
The WB3A is full of adjustments. Pop-off pressures, inlet track length, carb pull techniques, and fulcrum arm height all play important roles. Unfortunately, that is well beyond the scope of this article. Consult your engine builder if you desire more information on carb tuning specific to your engine combination.
Pipe/Flex
At last, the real black magic for all the members RLV’s pipe of the month club. To be honest, there is no one great pipe. Better yet, the new pipes may not always be better than the previous models. While it true new pipes are usually better than their predecessors; a good pipe is one that makes power where you need it. Often multiple pipes can produce the same power across the same RPM range when combined with the right flex.
RLV generally produces 3 pipes in each series (i.e. L-series, A-series). One is for low to mid range, one is mid to high, and the last is a narrow band (usually high range). Personally, I like a pipe with a broad power band so flex tuning isn’t as crucial.
So how do you tune flex and is it really that important? Flex tuning is pretty straight forward, yet it can be tricky since the gains are often small and particular track to track. Without going into wave dynamics and resonance flow, the general rule of thumb is a longer flex yields a narrow power band at a lower RPM. I recommend analyzing flex changes using split times as the changes can be minute. Concentrate on important sections of the track where the kart spends the most time (i.e. straights, long sweeping corners, corners that lead to hills.) Another great place to put your data logger to work.
The Package
Keep in mind, a strong KT100 package is tailored to a specific application (KT100 pipe, SSX Super box, Formula Y, etc.). Squish band, combustion chamber shape, and often port timing are varied depending on application. Running a motor built for one application in another application may not lead to optimum performance and possibly premature failure. Sometimes a motor can be reworked for another application. Consult your engine builder for specifics.
In the end, you are striving for a combination of gearing, clutch settings, carb tuning, and an exhaust setup which agree with one another. When you have a well-tuned machine, you’ll notice a different pitch in the engine, a maintenance free machine, and low lap times to boot! So don’t be afraid to try something, and document all your changes. Make notes of what works and what doesn’t. And last but not least, keep the wheels pointed forward.
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