4.3 liter engine bogs

[joebolt]

I have a 1990 vip 4.3 liter 2 bl. The engine bogs down when I give it power off idle and wont go over two thousand rpm. The engine runs fine on the trailer at all power settings. But when I put it in gear under a load in the water it just bogs down. I have ran a tank of gas through it and I have replace the points, condensor, rotor, distributor cap, coil, plug wires, plugs, carburator, fuel pump, fuel filter,and checked the fuel tank vent Also timing is set to the manufactures setting 6 btc at 650 rpm. I believe I have replaced all the fuel and ignition components and have hit a wall with where to go next. Any help would be greatly appreciated