1969 mercury 509 50hp falls on itself

Bought a boat and motor. Went through the lower unit put new no ethanol gas in. Put muffs on and fires right up sounds like it's good. Reved it up and engaged everything and cranks right up ( no load). Took it out and idles fine but when I try to throttle it up it wants to die and does. Pump the ball when it's at the point of quiting and it starts to come back but doesn't seem to have any power. Checked carb filters, hose and fittings for any leaks for air took the pump apart and check valves all looks good. Changed spark plugs and checked each for fire and they all are. It starts, idles fine and trolling speed it's ok but dies when given throttle. Any help where to start next greatly appreciated. Thinking I need to tear the carbs apart as next step?

Add 1/2 turn OUT on the idle mixture screws.

Jeff

Thanks I'll give a whirl!

Thanks Jeff night and day!
Any advice on fine tuning both carbs would be sweet
Apriciate the help!
Pete

The best way to adjust the idle mixture is to warm the motor thoroughly, tie the boat to a dock (or leave it on the trailer) and adjust for best idle in gear. Then add that 1/2 turn out.

Jeff