I agree with tIANci on the prop. We weren't too impressed with the performance of our 46AX when we first got it - followed all the instructions and ran 10x6 prop - after asking the question here on RCU, we tried an 11x5 and it sure seemed to suit us better - we now have a few of these motors on various models including a Blackhorse SU31 (which is probably the closest we have to your Cap) and they all run either 11x5 or 11x6 props quite happily (15% nitro fuel). On the 2 3d models that we've had them fitted to (Phoenix Future & Seagull Harrier) - they've had the 12.25 x 3.75 props as sigrun has mentioned.

I've recently "donated" all but one of my 2 strokes to the two sons (apparently) and now run nothing but 4 strokes in all but one of my planes - Saito 82 (or 72) and Magnum XL70 are a great way to go in this size model.