If that don't fix the problem, I'd be compelled to remove the bowls off the carbs, clean them and all the jets and try the external setup again.
If that helps, I'd then also be curious as how clean the stationary tank is and including any filters, vents, etc. And survey the condition of the hoses and fittings.
If all that checks out fine then you got to look for a fuel pump issue.
One step at a time.
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'75 Cuddy with '00 Johnson Ocean Pro 150 horse
Benny
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