Prop question

[lakejerome]

Changed from factory prop size 15 and five eighths X 12 (which *****ed about 3mph) to new prop size 12 and one quarter X 9 (part 663-45956-01-EL) which I ordered form boats.net in order to slow down the *****ing speed. Outboard is a 2011 40HP 4 stroke with fuel injection. New prop slows boat to less than 3 mph, which is ideal, but causes the motor to vibrate and shake too much at closed throttle. Vibration will only stop when throttle is advanced to about 3 mph or greater which was not an improvement on the orginal prop. How can I get the new prop to run at idle speed without causing the motor to run so rough? Raising or lowering the motor with the trim switch does not help.

[boats.net]

Make sure prop washer are installed correctly and prop nut is tight, torque to 29 ft lbs.