Is it the fuel tank or fuel pump?

Can someone give me an idea as to where or what to look at first? I just purchased a very nice 20 HP mercury motor for my 16' fishing boat project. The motor was used very little over the last couple of year, but atrted right up. It runs great and starts great, but when your wide open it starts bogging down like its out of fuel. If you reach down and pump the gar bulb it takes off and runs fine. So if i keep the bulb in my hand and give it a lite squeez it run fine. I looked over the tank and hose and they seem to be fine, but it is the steel 3 gallon can with the old tin cap and screw on it. Is it a bad vent, a leak in the hose, or do I just need a new tank? Or is it the fuel pump. I am mechanical, but have spent no time with an outboard motor. Thanks

Loosen the cap on the fuel tank and then run it, might just be lack of venting. I am assuming you are running fresh gas and have a CLEAN tank.

Replace the primer bulb and clean or replace the tank,better replace tank to motor.Clean the carbs for the practice or run a can of powertune through at least Graham says they're about $5,I wish,£12stg this side of the Atlantic

change the diaphram on the pump for piece of mind, then tighten all hose clamps as it might be sucking air instead of fuel. Agree with trying a different tank/line if one is handy

Ethanol is messing up a lot of motors.It's delaminating the inside of older hoses causing clogs in the carbs and fuel filter.Check for soft hoses and junk in the filter on the motor.Check the diaphram.
Check for water/crap in the tank.J

Where can I find Powertune?

Any Merc shop,I think West Marine stocks it.That's me guessing as I've only been in the one in Warminster PA. the couple of times I've been over there.Great steak house up the road from it.