Check my fuel pump logic here

My COVID project was to rebuild a 1977 open bow boat. It is on the water, but I am having one issue with the motor. When I first got it on the water, it did not seem to rev very high. I pulled the carbs and cleaned/rebuilt them. I also replaced the fuel lines and fuel pump. (a simple air-diaphragm unit).

It will now run at 100% WOT for about 30 seconds, then dies and chugs along. My theory is that the float bowls are filled at idle, then drain faster than the pump can refill, causing it to die. If you idle for a few seconds, you can go WOT again for 30 seconds. It will run 50% throttle pulling a tube all afternoon.

I further tested this theory by squeezing the primer bulb in the line while someone else was driving. (piloting? captaining?) It will run WOT all-day as long as you are assisting with fuel delivery. 

So my theory is that the $15 Chinese-made fuel pump from Amazon isn't up to snuff. I am buying a $20 one to see if there is a difference. The OEM pump is $100, but it is such a simple design that I can't imagine that OMC is doing something materially different than the aftermarket. Agree or disagree? Is it worth $100 to find out? 

OEM

Aftermarket

Is there any sort of screen or filter in the tank, or in the feed line coming to the pump?  Fuel line flows freely?

Check tank venting. Run WOT until issue starts crack open tank and see if it sucks air.

I think you've accurately diagnosed that you have a fuel delivery issue.  I'd check the tank vent a suggested and I'd also see what the recommended maximum hose length is and make sure you're within spec.

Personally I'd go with the OEM pump or at least one that has published specifications that match OEM.

Tank venting and float bowl venting are both easy checks. I'm not familiar with marine carbs, but I've had a few that had basically this symptom and it ended up being fuel/water plugging the carb vent tubes. The bowl pressure would equalize at idle, but couldn't when running with lots of throttle.

That said, I'm betting it's a fuel flow thing too, sucking the bowl dry.

Also check the fuel supply lines.  I chased a fuel problem on an old Johnson for a week and the fuel supply line was berkeleyed up.  Replaced the line and the problem went away.

Thanks guys, the supply line is brand-new but no clue on length. I will check on that!

The pump has a screen filter, but its clean. I may add an inline filter and remove the screen.

As for carb venting, all passages are clear from the cleaning no clue if there are vents or where they would be. I will research that.

If the new pump does the same thing, I may look at adding a small electric auxillary pump.