http://www.kxriders.com/forums/index.php/topic,4859.0.html
Lots of fun links...
Most interesting is the burn rates across combustion... VERY VERY similar... And then if you go read the lead amounts you know why they are slightly dufferant.
On iPhone at work... So enjoy my short answer...
Good info. Here is the article I read on AV pros and cons a few years ago, after reading the article again, sounds like you will be fine in a lower revving KX500 application :
Notes about AV gas from Harry klemm.
Harry used to do the motor work for Mitch at Pro Circuit, so he knows his stuff.
Tryce Welch told me much of the same a few years ago.
I run one gallon of VP C12 to 4 gallons of 93 pump.
"About Aviation Fuels - Aviation gasoline (or "av gas") is blended specifically for use in small aircraft. It's also commonly used by many high performance two-stroke owners because of it's high stated octane rating (usually 100oct) and the relatively low price compared to racing fuel. But it?s importance to understand some details about Av-gas.
Avgas octane is rated on a different scale than gasoline's intended for ground level use. What is 100 octane "av", is not necessarily 100 octane "ground level". Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.
Small aircraft are constructed as very weight conscious vehicles. That's because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, aviation gasolines are blended with no heavy molecule end. This makes a gallon of avgas weigh measurably less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a significant horsepower issue. Running 100% Avgas in a vintage two-stroke is not a good idea because there is a measurable loss of power resulting from the absent ?heavy molecule ends?
All that said, for most vintage two-stroke applications, a 50/50 mix of 91-octane premium, is a mix that offers a big boost in octane, along with ?enough? heavy molecule ends? to make good power. We consider it a very functional and inexpensive way to greatly reduce temperatures, and improve detonation resistance (over straight 91 octane unleaded). We would tend to recommend against it?s use in a sustained high rpm application (like road racing or pro-level 125s motocrossers). However in any vintage two-stroke that is not being run ?wide open all the time? ? it is a fuel that works great. It bears noting that some blends of av-gas will quickly separate from castor-based oils used in premix situations. If you are pre-mixing bean oil ? av gas is not a good idea."
