That little tube in bottom of the "air intake" is a 'sniffer port' so that if you get any drips of gasoline falling down from the updraft carburator they will quickly get 'sucked' into the intake manifold .... instead of collecting in the air intake (flame arrestor) - its a 'safety suction port' that helps the engine and boat from going 'boom' if the carburator starts to (very very rare exception) drop gasoline down the wrong way in the updraft carburator.
#1 Your symptoms sound similar to a dirty emulsion tube in the carburator. The emulsion tube is approximate 1 inch long by about 1/8" diameter and sticks down into the main jet well. There will be two teeny holes in the side of the emulsion tube and ***ANY*** dirt that gets stuck in these 'side orfices' of the emulsion tube will give symptoms similar to yours. That you cleaned and changed just about all the fuel components suggests that you might have dislodged some 'crap' and that its now stuck in these teeny side orfices of the tube. The emulsion tube controls the atomization of fuel before it gets drawn into the venturi section ... if the 'emulsion' (frothy gasoline) isnt correct you will get all sorts of 'fuel system' variability ... stalling, rough idling, rough running, etc. ... sometimes yes, sometimes no ... but never 'constant'. Simply look for teeny small flakes of rust, etc. in the tube and the 'side orfices of the tube', etc. ..... and be sure that the internals of the emulsion tube and its teeny side orfices are clear and 'spotless'. **ANY** 'crud' in the side orfices of the emulsion tube will yield a very rough and non-constant running engine.
#2. When you spoke with DonM did you discuss the 'match up' of ignition coil with your primary circuit in the distributor. You dont mention if you have the old fashioned 'points & condensor' set-up or a newer solid state (electronic) ignition. The newer solid state ignitions can not be used with the older coils used with 'points and condensor' as the coil will quickly overheat and malfunction
. Is the coil 'matched' with the primary circuit of the distributor????
#3 There should be a gasoline tank vent that opens to atmosphere somewhere on the outside stern of the boat. The tank vent *should* have a stainless screen
(spark arrestor) over the opening to the vent ... and the whole vent 'system' opening should be covered with a small metal 'cowl'. Spiders LOVE to spend their daylight hours inside these types of vents and their webs, nests, egg-cases, etc. will easily block such a vent.
#4 Whats the status of the *engine ground strap*. There should be a large braided stainless steel
'strap' from the engine to the main ground (ultimately connecting to the battery
negative). If there is any corrosion
or failure of the wire in the 'ground strap' or its connections due to years of accumulated engine vibrations or corrosion ..... the whole engine's electrical system
will be 'sporadic' until the 'ground connection' ultimately and totally fails. Check the electrical
continuity of the engine's "grounding (not bonding) circuit". If you dont know how to test for 'continuity', just run a temporary wire (8 or 10 ga. or "heavier") from the engine block (flywheel housing bolt, etc.) directly to the negative of the battery
and see if this doesnt "improve things".
Hope this helps.