The 91fx is a really good motor to run on the UCD. The only problem is propping it to get the transition and thrust you need from the throttle. The only other issue is all the weight you have to add to the right wingtip to balance out the reallllly heavy stock muffler. And if you use the stock muffler, youll have about 10 oz of goop on the wing after each flight. If I were to do another 91fx on my ucd, id rotate it about 30-45 degrees off center, so the exhaust points straight down the bottom. Maybe offset the head and the exhaust, so they balance each other out. And definately run a mousse can muffler on it. Less weight, better transition.


Alot of the guys have had great success with the ST90 and a mousse can. Ask dave mcdonald or m grambling. Theyve had st90s on for a while.