My 71 18hp evin when under full throtle its starts slipping and will act like its in neutrl as soon as i bring it down to an idle it will take off again???
you have either a spun prop, or a broken drive (shear) pin. it is a metal rod that goes thru the prop and prop shaft connecting them. usually broken when the prop hit something, or the motor is shifted in to gear and high revs. remove the prop, and see if the pin is broken.