I had a similar problem with my 1970 StarCraft w/ 140hp Suzuki, gave me fits of trial and error! Limped back to dock w/ friend constantly squeezing primer bulb! So starving for fuel??
Long story short (after going through all 4 carbs carefully myself, and was not the problem) I removed Suzuki quick connect fuel line from top of fuel tank. #2. took a wrench and removed male quick connect fitting, turned over fuel line fitting and looked into treaded end, looked like black debris up in there. Inserted a piece of wire in to it and came out with BLACK GUNK! Where in the hell did this crud come from?! It came from the inside of the old black rubber original equipment vent hose... slow deterioration dropping rubber particles into fuel tank. Removed fuel from tank and filtered, cleaned out fuel tank.
Replace or clean fitting, replaced ALL fuel line from tank to engine and between carbs. Some carbs have plastic fuel fittings, be careful. I used a razor knife and cut a slit in fuel line over fitting, then gently remove.
Replaced vent hose. Problem over! : )
BTW I have never used regular fuel w/ 10% ethanol. This was just an old age fuel/vent line issue. However, as you might already know, use of 10% ethanol fuel WILL ACCELERATE this kind of fuel system mess for a number of reasons on boats, all small engines also!
Some gas stations in my town sell NON ETHONOL premium fuel, or I buy at the fuel dock.