Mercury 15 hp 4 stroke driving me crazy

I bought a 13 ft Boston Whaler with a 2007 Mercury 15 hp 4 stroke long shaft on it.

While it would start right up, when you increase the throttle, the engine dies.

Seller replaced fuel lines and bought a new fuel pump.

So far this has been my experience:

Took delivery at boat ramp, ran at slow speed for 10 minutes (speed zone), then opened it up. It ran fine.

Later same day, took it out. Ran at slow speed fine, when I opened the throttle up, it died, and I couldn't get it to restart. Came home on my electric motor.

Replaced fuel pump. Ran at slow speed fine, ran at WOT fine.

Later same day. Ran at slow speed fine, initally ran at WOT fine, then died, couldn't restart (electric motor home again).

Noticed that fuel bulb would be firm, and then when I tried to start it, if would become squishy.

Anyone have an idea on where I need to start. I'm assuming it is some kind of air leak, but I'm not sure. I've checked the hoses and connections, put hose clamps where there were none, and still have problem.

Not that familiar with 4 stroke, but you need to determine if you're losing spark or fuel. Sounds like fuel. Start with carb cleaning/rebuild, fuel filter, fuel tank bulb. Make sure fuel pump was re-assembled correctly. Does it keep running if you keep squeezing bulb or spray starting fluid in the carb?

Thanks for responding.

Fuel pump is new, hoses and bulb are new. I pulled carb this afternoon and cleaned it. Ordered a rebuild kit and will see if that does the trick.

Not familiar with 4 strokes at all. But if I was you I’d check for spark on all cyl. Then check fuel.
if it’s fuel related it could be a dirty fank

Update.

I replaced fuel pump, carburetor, and fuel line/bulb from tank to engine, and fuel line from fuel pump to carb.

Took it out yesterday: Idled fine, ran WOT fine for 30 minutes.

Took it out again yesterday: Idled fine, when I advanced throttle to WOT, engine bogged down and almost died.

Took it out today: idled fine, and when I throttle up, it almost died again.

I checked gas tank take up pipe, made sure it was submerged, and pumped bulb. Gas doesn't flow from tank when bulb is pumped, and bulb doesn't get hard.

If I wasn't already bald, I would have torn my hair out by now.

If the fuel line bulb does not get hard, you have identified a problem.

If the arrow on the fuel line is pointing towards the engine and the fuel line is new, try a different tank.

Otherwise, could be the carburetor inlet needle is not seating (but, you say there is no gas flow).