Your .25 engine should be OK with an 11x3 prop. Someone on the tach forum got 12,500 rpm with a 10x4 on a Mag .25. Of course, it will get hot and stop if it is too lean . Put on a 10x4 or 9x6 and see if that will run ok. Use a tach to see how it revs. If you get less than 12,000 rpm it is probably too much load for your engine. If the 10x4 is too much try a 10x3 or or 9x4. These props are dirt cheap and take a few minutes to try, so give it a go. Make sure you have the engine rich enough before you proceed.