1985 Johnson no power when full throtle

My brother has a 1985 Johnson 140 vro and it runs great at idle and even full throtle, but at full throtle it don't seem to have any power to move the boat. The rpms are low and it don't sound like the motor is running at its max. We have changed the plugs the wires the coils the fuel lines and still nothing. When he first got the boat it ran like it is now and he changed the plugs and it ran perfectly, we got 2 runs out of it and it went right back to its sluggish no power self, any ideas for us. Thank you

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