I assume that the 17-4 Fun Fly is a 17-4W. If so it would be the equivilant of a 17-6, which would work fine. The 16-8 and 17-6 are the recommended props from people I have heard from in these forums. I used a 18-6W to break in my Saito 180 and I could only get about 8500 RPM out of it at 2 3/4 turns. I'm pretty sure it was too much prop but I didn't really know it at the time I bought it.

What prop are you running now and what RPM's are you getting for peak? Also how many turns out are you on the high needle?