Your original post said you were having issues with the .55, but were considering making repairs. You said you'd consider using a .75 you had laying around, but were concerned about weight, and problems you may have with that engine.

Now you're talking about buying a .65AX?

The AX line of engines gets great reviews. There are always haters, but basically these are engines that provide adequate power (not the most, not the least) and offer a ease of use and little-to-no break in time. I would be confident that I was getting a good engine if I were to go with the .65AX. If something were significantly wrong with the motor, you'd be backed by great support.

That being said, it seems like you are having unfortunate issues with two of those engines. To me, it sounds like there is something wrong other than the engines. I'm not sure why you'd want to get a third before you have the first two figured out.

I think the best advice is to repair the .55 and figure out what's going on with your engines. It is less expensive than buying a new engine (and with only marginally more power) and will hopefully teach you something along the way. That engine will be enough for aerobatics and some 3D, depending on how you prop it.

If you're concerned about performance, a good 4-stroke is always an option as it gives better throttle response and generally allows for a larger diameter prop. However these can be more complicated than the AX engines, so maybe not a good idea yet.

http://www3.towerhobbies.com/cgi-bin...?&I=LXWFK1&P=0