Sounds like a classic fuel starvation problem - ok at idle, but bogs down when higher flow rates are needed. Using a remote tank as suggested is a really good way to determine if it is engine related or the fuel delivery system. If it is still an issue on a remote tank, then it is the engine.
See if the problem goes away while you squeeze the primer ball - if so, you likely have a clogged fuel pump screen or bad fuel pump. Check the screen at the fuel pump, check the fuel pump diaphragm itself, and then look to rebuilding the carbs as there may be issues with the high speed jets being clogged, or possibly timing advance issues - google link and sync. You might be able to borrow a tank from a fellow boater in the area with the same engine connectors.
If the problem goes away with the remote tank, then something upstream of the motor in the fuel system is giving you problems. Any of the following are causes of your exact symptoms:
1) old / contaminated fuel. Disconnect hose at engine and pump some in a clear glass jar - look for cloudiness, water settling to the bottom, sour smell). If it looks ok, then this may not be your problem as originally thought.
2) plugged tank vent line. Unscrew tank filler cap and if the problem goes away, it is likely your vent line - sometimes insects build nests in the hull fitting or hose.
3) collapsed fuel line. If the lines are old or original, they may not be holding their shape and are flatened out restricting flow. The original lines in your boat were not the newer ethanol friendly lines.
4) plugged in-line filter. clean / replace as needed. I'd strongly recommend buying a Racor fuel water separator and mounting it just before the primer bulb. It will catch water and crud before it reaches the engine. No need for cheapie in-line filter if you have this.
5) bad primer ball. Give it a few squeezes - it should maintain its shape and get pretty firm.
6) clogged fuel tank pickup. Many have a screen at the bottom of the pickup and if partially clogged then it will cause fuel starvation issues.
The advice is mostly free, you need to post back your findings. And pictures of your boat. Welcome!
