I'm getting about 5900 RPM's at WOT. I should be at the 5400 RPM based on the Manual. It's a ELPTO 90 HP 2005. Current Pitch is 17, don't know diameter. I need to bring down the RPM's so am going to go with a 21 but, don't know what to do about the diameter. What would be the correct diameter for this pitch? <br /><br />Thanks