I have a 1978 305 OMC sterndrive on a 17.5 ft Slick Craft. At WOT I'm hitting 5400 RPM's. I believe this is too high but what should my top RPM's be. Prop has the #19 stamped on it. Its a little slow getting out of the hole with a full load but other than that operates well. Could the tach. be...