going to build ultra 40 just need to no the differance between 2 and 4 stroke motors

If you want the Ultra Stick to perform well and be able to hover and to all the aerobatics its capable of, then you'll probably want to go with a Saito .72 or YS .63. I have flown the US 40 with a Saito .72 and it gives great performance. I have also seen many fly with .46 or .50 2-strokes and they don't have enough power for things like hovering. The only 2-stroke that i see hovering the plane is a Jett .60 and its more money than a 4-stroke. Plus the 4-stroke is quiet and won't cause any noise problems at your field.

I had a Saito .72 on my US .40. It flew quite well with it, but it had a weird problem which would cause it to go horribly rich and shut off abruptly after running for extended periods at half stick. I had a couple of crashes because of that.

I am now running a YS .63 on the plane with a 13x6 prop. So far this is my favorite airframe/engine combo... it can do every stunt I know (and some I don't ) and runs perfectly.

You may have just had a bad engine. Right now the Saito .72 I have has been running great for the last 6 months without being touched. I have yet to even see a YS .63 run, but I really want one.

It is weird. Sometimes it would run perfectly, then suddenly gurgle-gurgle-sput-hic <silence> This even happened during a tethered ground run!

I probably need to just run some more fuel through it, as I don't think I've run it more that 15 times.