Most engines today won't let you over-rev. They have rev limiters set for max RPM. Check your manual and see if you are on the limiter. If so, go up a couple inches in pitch. You didn't say if your Zuki 175 is a V6 or inline 4, but I think both go on the limiter at 6,000 RPM. The inline 4 should run about 5500 RPM WOT with the right prop, and about the same for the V6, although the V6 has a wider power range and you could be as low as 5,000 and still be OK. Consider a higher pitch prop for sure if you are running above 5500.