John, the engine in the picture is s downdraught, the carb however is a standard Mikuni updraught slide carb. However that does not change the way the intake power tuning works. I also worked on 125cc twin Hondas in a go-cart with open exhaust pipes of tuned length. Being OHC engines the carb tube is high! In this engine as well, using shorter intake tubes caused quite severe power loss. One has to consider here the 2nd, 3rd and 4th order resonance. First order is very long, and very peaky. IIRC, the 3rd order length was 75000/rpm (inches) rule of thumb. The higher the order, the less gain, and the less peaky.
BTW, The calculator also has intake duct length as output, and responds to engine cam timing well.

Edit:
some corrections: see http://www.irday.com/html/Engine%20f...0412/8777.html
I was wrong about the harmonic order. Here are the correct data
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Probably the single most important induction parameter is cross sectional flow area, versus cylinder displacement.

There will be an optimum flow velocity that produces a torque peak somewhere in the Rpm range. Below peak torque velocity cylinder filling poor. Above the peak, pressure drop will reduce cylinder filling.

This peak can be moved around somewhat by also tuning the runner length, but the peak cannot be moved very far doing this. If the valves and ports are far too small, the engine is not going to make massive top end power, no matter how short the runner length is made!!
Pressure waves in the intake runner can be used to shift the torque curve around, and the intake length can resonate at more than one Rpm. It can be tunes to a different harmonic.

Second harmonic tuning is the strongest where length in inches = 108,000 divided by Rpm.
Third harmonic tuning = 97,000 divided by Rpm
Fourth harmonic tuning = 74,000 divided by Rpm
Fifth harmonic tuning = 54,000 divided by Rpm.

While second harmonic tuning is the strongest, you can only get one peak, and the runner length can be inconveniently long. It works best on very peaky high Rpm engines.
On a sports car something much shorter that uses fourth and fifth harmonic tuning is better because it will give a better spread of torque and be less peaky. For instance a ten inch runner will peak at 5,400 7,400, 9,700, and 10,800 Rpm. The peaks get higher at higher Rpm.
Having a double hump at both 5,400 and 7,400 might work rather well on a road car. It might also work well on a motorcycle engine peaking at 10,800 Rpm.
But tuning the runner length is only going to be effective if the induction flow area is sufficient to support enough flow at that Rpm.
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