I just got the u-can-do-it 3D 60 size plane and the only one way u can mount a 4 stroke is inverted. I am using a Saito 100 with no pump, will this engine run fine inverted???? Paul

paul.
all of my 4 strokes are mounted inverted and I have never had a problem. Though mine are all YS and OS. As for your particular engine a friend of mine has the U Can Do with the saito 100 inverted and runs just fun. Besides, with this airplane how often will the engine actually be inverted

My buddy has a Saito 100 mounted inverted in an H9 Mustang, and it runs fine. All my four strokes are also mounted inverted without any problem.

You shouldn't have any problems with the inverted four strokes. I have used the OS 91 and Saito 72 inverted. The engine doesn't have any idea which direction it's facing, however the only drawback I've found is trying to reach the glow plug. If you have a cowl, I suggest using a remote plug or onboard glow. If not, you just have to reach a little more during starts. You might also look at using a single point fuel system inorder to keep from flooding the engine.

I think regardless which mounting method you use, the engine break-in and needle valve tuning is much more important than which way it's mounted.