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I had the exact same motor on my Montauk 17'.
I used a 15 pitch OMC prop but I also had a 17 pitch that I used once and awhile which also gave me a spare.
Make sure the motor is mounted at the proper height, not all the way down. Good props for your motor would be the old omc 13" x 19P sst mounted 2 holes up or the stiletto 13 1/4" x 17p mounted 3 holes up. Different prop by different manufacturers will have some variation in which pitch works best.
Doing some research it appears the 1985 Evinrude 70 had a redline of 5500 RPM and the 2.42:1 gear ratio.
If this is what tdeary has, he will want a 13-7/8" x 21" BRP Viper, a 13-1/2" x 20" BRP Raker or a 13-1/4" x 19" Stiletto Advantage 4.25.
In 1986 OMC redesigned the 70 with a bigger engine block and a 6000 RPM redline. One would have to drop two inches of pitch for this motor to work on a Sport 15.
