Here's my two cents worth on the "bad gas" legend. I suspect that many of the volatiles in the gas boil off while in storage and the quality of the gas does deteriorate. My guess, and that's all it is, is that this effects the combustability of the gas. By that I mean it does not burn as efficiently in the combustion chamber and hence produces more by-products, most of which I would think are carbon.
I've seen weedwhacker engines that had the rings glued in place with carbon. In talking to all the local lawn, chainsaw and farm equipment mechanics, they say it is from running last year's pre-mix. If you think about it, if my "guess" is right, when you combine that with oil, and let's take it one step further and say cheap, Kmart oil, you probably produce way more carbon than a fresh batch of gas with good oil would produce. But in a 4 stroke, unless you run old gas as a regular diet, I suspect that the negative effects are minimal.
As a sidenote, I bought a new SeaDoo back in 1995. The dealer was changing owners and the previous owner, who was the original dealer for SeaDoos in the US was working with the new owner as he got up to speed. When we finished the deal, the original owner, who was quite the motorhead and very creative guy, invited my wife and I to his house to see a few of his toys after we made the motorhead connection. While at his house, he told me to only run the SeaDoo oil in the engine. He was not going to sell me anything and even if he was not selling the place, his dealership was too far away for me to buy my oil from him. He said that the carbon residue left behind by the other oils, even name brands like Mercury and OMC outboard oils, was too high and would ruin the performance of the Rotax two stroke. He said most of the engine problems on those engines while he was in the business could be traced back to using the wrong oil. The point of this is the carbon deposits are probably the most pronounced negative effect in burning old gas.
One more whack on this horse just to make sure he's not breathing. Farmers have equipment on their farms that can sit for years without being run. My friend Dave has tractors with trees growing up around the axels. Most are very old gasoline engines. With almost 100% confidence, you can put a new battery in them and they will fire right up and run like a top and continue to run well. So, again, in 4 strokes, I don't think bad gas is an issue until you get into very high performance engines requiring high octane content.
Wait, I still have one more swing in me. I would be willing to bet that many of the "bad gas" problems were caused by dirt and other junk, like rust or water in the gas tank that plugged filters, carb jets, or just filled the float bowl with water.
One last two stroke story from this winter. My bro-in-law who is not very handy, brought his one year old Sears weedwhacker to me at Christmas and said it would not run last summer. I figured it needed a diaphram kit or the screen was plugged in the Walbro carb. But that was not the case. The fuel pickup in the tank has a porous stone filter. The gas/oil mixture, and maybe it was the E10 gas alone, produced a deposit that completely coated and plugged that stone filter. I put compressed air into the fuel line and blew into the filter and this goo slowly emerged from the filter. I soaked it in acetone and blew thru it again and it cleared the rest of the goo out. I suspect the goo was the phase separation we read about. I'd never seen that before.
Gee, I kinda got on a roll there, didn't I. Nothing like the first cup of coffee to get the ranting juices flowing. See what you started, Jim.
regards,
Rob