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.

Wow, you had an interesting few days.

They are pretty tough engines and as long as the oil was still getting to the engine you should be fine. Personally in my YS engines I have found that Wildcat 20/20 Ys blend works well and if you can afford it and really want power use Coolpower 30% Heli which is 24% oil. If you have a Hobbytown close by their 20/20 is Wildcat relabeled but at the Hobbytown close to me it's 40 per gallon and my LHS has it for 22 per gallon

You probably already know that YS engines love nitro and more is better in these engines. I've seen guys running 45% in them. [X(]

I was/am running wildcat...

I'm thinking somehow that gallon didn't get it's full 20% nitro, it would run full throttle, but got hot and started dropping rpms, it would also run mid, but wouldn't idle without the glow starter... just shut off like you shut it off on purpose. The scary part was why was there no oil on the wing? dunno... hopefully it wasn't the oil it was missing... I know that thing got pretty damn hot a few times... I just hope I didn't cook the whole engine..

bill, i would prefer something other than hobbytown due to their extremly high gas prices..

camss69

I agree, it sounds like it was low on oil and you may want to either pull the back plate off, or the very least pull the head and take a look at the pistonand cylinder. You already know what you are doing so I will not suggest what to look for but I think I would be safe and send it to YS for a checkup. As you know they are well worth it. Light weight 4 strokes with gobs of power.

Newb.

I know about Hobbytown and I also know about your experiences with them. I personally do not go there as I work for my LHS so I get my goodies at cost plus 10% but I will suggest them if there is no other choice or I do not know what is close to a member asking a question.

Sorry to hear about your problems with your YS. I hope that you didn't do any damage to the motor, but if you did you can get it sent and have it repaired. A YS is definately worth repairing. From what you said it didn't sound like you ran if very long in that condition, so hopefully it didn't damage the motor.