1987 25 HP YAMAHA - No power at high rev.

The motor starts great, idles great and revs up and down with plenty of snap, when in neutral. I can put it in gear and it still runs perfect and I can troll all day. When I hit the throttle the motor bogs down and I can't get any power out of it. It will intermittenly all of a sudden give me the power but it never last longer then a couple of seconds. It sort of has the feeling when the prop is tangled in weeds. This only happens when the motor is under load. If I hook it up to the garden hose I can put it in gear and rev it up and it works properly. Anybody have some advice?

clean the carbis ,and check the main jets with wire thru the hole.try new spark plugs.check the wiring to the coils .try turning the idle mixture screws out a quater of a turn.that improved my 30hp yami