Air-Fuel Mixture Analogy for Debuting Scooter Tuners

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Wheelman-111
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Air-Fuel Mixture Analogy for Debuting Scooter Tuners

Post by Wheelman-111 »

Greetings:

Recent experimentation with my own scooter and a question from a Forum denizen prompted the following thoughts regarding fuel/air mixtures. All engines require a well-matched mixture of fuel and oxygen for effient combustion. Disregard the fact that air is 79% Nitrogen, only 20-ish percent Oxygen - it's built into the jetting calculation factors. Anyhow something around 15 parts air to one part atomized fuel burns best. Note that air flows a lot easier than liquid. (D'uh!) :)

The running Honda 2-stroke engine is a source of vacuum - AKA negative pressure - on the intake side. Imagine holding two straws in your mouth. One small one is dipped in liquid. (Say a nice Single-Malt Scotch:) ) A much larger one is open to the air. Your lungs are the bore, clamoring to be filled with... something. Your mouth is the crankcase where mixture gets ready for the Main Event. Close your gaskets - er, I mean lips - around both straws and inhale. If you pull hard enough there'll be a little spatter of hootch hitting your tongue, but a lot of air.

Now pinch off the bigger straw just a little. You could put the end in a sponge saturated in oil to produce the same effect, just like Honda-san. Repeat the inhale with the same intensity. Didja get a nice spray of the rotgut? Good! Now you know how filters affect mixture.

Pursuing this analogy, try a bigger straw in the booze. You know you want to. :) With the same intensity of inhalation, you may find a good slurp of the stuff in your mouth. Rich mixture.

Now vary the force with which you inhale. A light slow draw = low RPM or smaller displacement. Not much satisfaction from the liquid side? Lean mixture. Need a third, and perhaps a fourth straw, maybe make them adjustable somehow. These are your Pilot and Midrange circuits found in many carbs.

Now back to two straws. If you inhale really hard, unlike a certain former President, lots more air will draw in for the quantity of liquid you receive. Bigger bore or higher RPMs require a bigger jet, er... straw to match the incoming air.

I'll defer the discussions regarding fuel "watered down" with premix oil, akin to adding - GASP! :shock: - water to the Scotch. Likewise things get complicated with variations in altitude (thin air acts like less air...= rich) - temperature (colder air acts like more air = lean) and humidity. I just wanted to simplify the air/fuel equation to terms I can understand. I like this so much it's going into Tech Documents.
Wheelman-111
Most of my money is spent on scooterparts. The rest is just wasted.
"ISO": '03 Vespa ET4 Malossi187 74MPH
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox/Carb/Pipe 58.8 MPH
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