1996 mercury 50hp 4 stroke bigfoot stalling

I bought this pontoon boat last year. I can run this thing for days with no problems. Then bam hard to start and bogs down. I have to start it and hammer down the throttle to get back to the doc. If I let up on the throttle it dies. It’s been in the shop 3 times. Carbs rebuilt. New plugs, gas, fuel pump tested fine. This last time they put in a new throttle cable. And today the 5th time out bam it started again. I’m so stumped why this is happening. Anyone have any ideas. I’m tired of putting it in the shop with no luck

External fuel tank?
Is it vented? When it happens is the tank blown up like a balloon?

Ive seen fuel lines internally separate and collapse, new line maybe?

It’s a built in tank. No vents. I let it sit with outboard level and ran just fin going to the doc. Maybe it’s when I’m in shallow and I fish with the outboard tilted up

Is it only running bad when the motor is trimmed? If so maybe the fuel hose is kinking?
DT

Well it’s hard to explain. I will try. I can run it for days at a time with no problems and then bam it is hard to start and when I get it started and push on the throttle it stalls. Then I have to get it started again and hit the gas quick to get back to doc. As soon as I let up off the throttle it dies. I have ran it all week and today it did it again. I limped it to the next spot. And I was thinking I had the outboard tilted up because I ran the boat up on the bank. So this time I just anchored. And left the outboard down. I was thinking everytime I find a shallow spot I tilt it up while I’m fishing so i done beat the prop up moving about. I dont fish like this much. Thinking back everytime I fish like this I get that problem. Today when I left the spot I was anchored at with it down in the normal running spot it fired right up and ran great. I still dont understand why this is happening

First make sure you don’t have any of those crappy fuel lines with the plastic liner.Especially the one from the fuel connector on lower cowl to the fuel pump.

Also if it always runs fine when vertical,but dies when tilted,it could be incorrect float settings.If it blows black smoke,it is too rich.If none,it is probably going too lean.Those carbs are very fussy.Next time you run it,start with motor vertical,then slowly tilt it.If it dies when halfway tilted,then the carbs need more attention.

I second the fuel line possibility suggestion. I have the exact year of outboard on my pontoon boat. My main problem was the engine would push the boat fine for about 300 yards at full speed then bog down and only run smoothly at half throttle or less. I rebuilt the fuel pump and replaced the fuel filter. It turned out to be the fuel line from the built-in tank on the pontoon boat. It was blocked by varnish-looking strands in the line. To easily test it, put some fuel in a separate portable tank and use a different fuel line to test the boat. Having done the test and replacing my fuel line, it has worked great ever since.

Yes, try a remote tank. Clean it out first.