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Make sure the packing nut/plastic bearing for the low speed needle is there. Easy thing to overlook on those motors (early 70’s thru ’76 models of 18-20-25). It will present in the same way you are seeing. If it is missing or you don’t have one, you can smear some grease or perhaps some vasoline (that will incinerate inside the motor since it’s petroleum based, and you can get it out with carb cleaner if necessary) in there temporarily to see if it helps, but will need one eventually.
Make sure your float is set correct. Level with the upper carb casing, and make sure it wasn’t accidentally install it upside down – the motor will just plain do weird things with no rhyme or reason. Of course, make sure the carb is clean without obstructions through various orifices.
Have you noticed that after it stalls, the carb needs to be re-primed? If it does, then you know it’s a tank or pump issue. If it is flooding as you prime it, well then your float, needle, or boss gasket is bad (or maybe all of them).
You may want to try running it with the choke closed to confirm it is simply a fuel metering issue at the carb, not delivery. If it runs longer before stalling, or at least runs albeit incorrectly, then it’s a metering issue with the air/fuel mixture.
Otherwise you have an air leak somewhere of course. Just be advised that the low-speed needle is just about useless for adjusting things on those motors. I’ve seen them range from 1.5 to 2.5 turns out to get them to idle better without a cough, but they will smoke like hell and blubber badly no matter what you do. Run it too lean and they spit back badly; basically all you can do is tighten the needle to the point where there cough, then back it out enough so it doesn’t but also doesn’t totally blubber out. There’s almost no leeway between the two extremes, either. (thank you very much, stubborn OMC engineers…not listening to the service reps 😀 ).