Marine carbs have 2-3 fuel circuits. One is the high speed circuit, where fuel is supplied from the bowl thru the HS jet. It is sucked up from the bowl thru the large pickup tube by venturi effects as air flows thru the carb throat.
Once you get past say 2500 rpm, the majority of the fuel is flowing through the high speed circuit.
But, idle and midrange is a different story. In most marine carbs, idle fuel is supplied to the engine through very tiny holes drilled thru to the side of the venturi, opening up right where the butterfly rests when closed. Look in there and see the holes in the side of the venturi.
Idle fuel is sucked up the very tiny brass tube that extends down into the bowl. Sometimes that small tube runs inside the larger HS pickup tube. Other times
it is off to the side by itself. These tubes are smaller at the top than the bottom. If those idle fuel tubes are clogged, the cylinder will not idle properly.
Lean sneezing, etc. and in your case, stalling. You are supplying the missing fuel to the motor when you hit the key to activate the primer solenoid. Fuel comes in from that source, instead of thru the possible clogged idle circuits.
Metal carbs have the idle fuel supply holes drilled directly into the carb. So, when someone "does the carbs" on a metal carb, the small idle holes are, by default, right there to be cleaned....if the mechanic is good.
But the plastic carbs have been designed differently. They cannot hold such tight tolerances in the drilled holes when drilling in plastic. So, the idle fuel circuit is moved from the black plastic carb body out to the throttle bodies. Those small holes live there, right where butterfly is.
On your motor, you will find those holes behind a round knockout plug on the side of the throttle body. The cavity behind the plug is called the calibration pocket.
On all plastic designed carb systems, it is crucial to clean the throttle body. It is part of the total carb system, and is often overlooked because it is physically a separate part.
I use welding tip cleaning wires to get those very, very tiny holes opened up.
Before tearing it down, tho, you can try the spray carb cleaner with a straw trick. Put the straw directly into the idle air bleed jet in the front of the carb. With engine running, spray liberally. Do all four carbs, use the entire can. See if it "fixes" it. You MIGHT get by this way.......
EDIT:
I just looked at the parts pages for your engine. I see the throttle body has a plate held on with two screws instead of the round knock out plug described above. This makes it easier....
