In my experience neither the .25LA or .25FP will pull a 10x4 or greater prop with any performance. I have bought just about every prop in its range and found the most thrust happens with a 9x4 or a 10x3. 9x5 adds a little speed and sacrafices thrust. 9x6 is borderline too much prop for performance but does quiet the rpms down. 9x7 would kill any bushed .25 that I have seen. You would really have to lean the engine out to get a 9x7 to pull and I think you would burn it up before the season was over. An 8x6 inch prop will work but it won't give you maximum thrust. If you really want to know how your engine feels you have to tach it. You can tell alot just by flying it but you won't be certain till you tach it. It wouldn't hurt to borrow someones temp gauge to see if its overheating with the 9x7 which can also cause premature engine wear. I typically like to see my 2-strokes in the 12k and up rpm range for bushed and 13.5K and up for ball bearing.