I say lesson of the week because it's taken me a week to figure out this problem. I had a perfectly running YS120 one day, and was on my last tank of fuel out of my gallon jug. I dinged the prop on the landing and pumped the remaining fuel into a brand new gallon of fuel. Packed it up for the day, rebalanced the prop and fixed the tweeked gear, and was out the next day to fly. DAY TWO, got the engine running and realized the thing wouldn't idle, no problem bad plug, change plug, run up engine, and in the process step on the stab and break off the left side. Couple days later, fixed stab, fuel up plane, no idle, change plug, no idle. I also notice that the needle valve is very unresponsive, and notice the engine isn't spitting as much oil onto the wing as normal. Must be a bad fuel line, nope, try a new plug, nope, pull off cowl, must be clogged needle valve, or stuck regulator.... tear them both apart and clean them both out. Reassemble, still no idle.... Fuel pump dies on flight box.... trip to hobby shop... pick up new plug (I've gone through the three i had) foam rubber, and just for grins another gallon of fuel. Got home this evening and try it again... still no idle. I'm about to take the engine and box it up to send in, and I figure well I might as well try the other gallon of fuel just so I can say I did it... . idles perfect.. The lesson here is that the only thing that changed from when it ran fine to when it quit running was the fuel I know better and should have realized this. I tell you I tried like hell to get it to run on that bad gas, hopefully I diddn't jack up a perfectly good engine. I figured there was no way a brand new gallon was bad but it sure was. I've been using the same brand forever and that's my first bad gallon, of any fuel for that matter. So now I'm going to have 2 bottles at all times, and remember if it goes bad on the first tank of a new gallon, check the fuel.