mercruiser 4.3 stalls after shifting to forward

by winston
(perth amboy, nj)

i have an inboard mercruiser 4.3 lx i have no problem turning on the engine and increasing the rpm in neutral, but when i put the boat in the water and shift to run it, it stalls on me, i can put it back in neutral and start it right back, and shift again and stall. any idea thanks in advance