The Air Cooled Two stroke Jetting Dilemma.

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And just when you think you are getting a handle on the topic:

"The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 15:1[1] i.e. for every one gram of fuel, 15 grams of air are required. The fuel oxidation reaction is:

\frac{25}{2} \mathrm{O_2} + \mathrm{C_8H_{18}} \to 8\mathrm{CO_2} + 9\mathrm{H_2O}
Any mixture greater than ~15 to 1 is considered a lean mixture; any less than ~15 to 1 is a rich mixture – given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles that use an oxygen sensor or other feedback loop to control fuel to air ratio (lambda control), compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition and controlling fuel volume. Vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain fuel blends (especially winter fuels used in some areas) and may require different jets (or otherwise have the fueling ratios altered) to compensate."

From Wikipedia. Admittedly an open source and so not necessarily vetted by the SAE, but it sounds familiar and is probably right.

Note: I notice the math symbols didn't translate. Bunch of lambdas and whatnot. Rather than try to get the translation right, just go to Wikipedia.

[racerclam]

Just a word of mention . The Intelajet available from thunder products ( THE MAKER OF THE UFO) works really slick and is remotely adjustable from where ever you mount the controlled air bleed and another plus is that it discharges atomoized fuel not raw fuel. I have sold and install several of them with good results and pleased customers . One of my shifter cart racers just watched his EGT gauge and turned the dial intil it read his desired temp of 1050

Rich

[taber hodaka]

Some new chain saws high tech, computerized and all are giving shops fits. Every thing checks out but it wont run?? The customer, mechanic and the shop are not happy, only the competitor. ------------Clarence .

[hodakamax]

Greg, there may be more than I need to know about stoichiometric mixtures in your last post. I'm still trying to figure out how the TV works.

Max

PS--Interesting, almost everything can be made to be confusing. Fun discussion though!

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Let me de-confuse best as I can. The ratio that is physically/chemically the most efficient causes all of the fuel to burn. That's 15:1 using unobtainable gasoline. The farther away from 15:1 you get, the less efficient the burn. But efficient burn isn't necessarily what you want or need. Opinions vary on what ratio is "better" or more sustainable, with most agreeing that something in the 12.5:1 to 12.7:1 range is just fine. Not too hot, not too cold. Just right. As I said, it is more of a median ratio than an optimum or ultimate target number. Numbers in the mid twelves are relatively easy to accomplish, and you probably do it all the time. I have no idea how successful your average tail pipe sniffer is at telling air fuel ratios in a two stroke that not only doesn't fully burn any fuel charge no matter how ideal, but also has burned and unburned oil in the exhaust, so I don't know who well actual AF ratios can be determined.

So I think the numbers are far less important than how well an engine runs when put to its principal task. If it is running well, making good power, not burning parts up, not seizing, not fouling plugs every day, and not puking goop out the pipe, I think that's pretty close to optimum, whatever the ratio is. Gains from running leaner than running just fine are probably marginal at best. There are other and easier ways to gain power than tweaking jets to the verge of engine failure.