Hi all, I'm looking at upgrading to more power on my goldberg ult. bipe from the YS91AC to the YS120NC. Iput in a smoke pump, tank, and battery and now it doesn't fly like it used to. Is the 120 the same physical size? And could Iexpect a moderate difference in performance?

Looks alike you have 2 questions. I don't have experience with that particular airframe but I do with YS.

1. The YS91 and YS110 are roughly the same size/weight. The YS120 and YS140 are the same size/weight (about 5oz heavier than the 91/110).

2. I suspect that the extra weight added with the smoke system is causing the flight differences. Adding more power won't make it fly right.
Smoke just isn't practical in very small airframes because of the weight. If you just needed a bit more power then the YS110 would be the choice.

If you move from the 91 to the 1.20 you will need to change the mount as the 1.20 is a larger engine and weighs more. It will certainly give you more performance but realize that you are going to increase your wing loading even more. With the addition of your smoke system you added weight and added to the wing loading, the 1.20 will just add more. A 1.10 weighs little more than the 91, drops into the same mount and makes almost as much power as the 1.20F. If you already have the NC then I can see using it but if you were looking for a replacement engine the 1.10 would be my choice.

Thank you both, I knew that some of the engines were the same physical size...I just didn't know which ones. I may try looking for a 110 since it will fit right in.