i see a combination of possible issues here.. mainly in carb settings.. why factory pipe would recommend 2.5 turns out on the idle screw is crazy.. even in the mikuni tuning manual, it says if youre passed 2 turns out, go up in jet sizes.. essentually what is happening here is overly rich low speed screw to compensate for lack or pilot jet will cause off idle rich condition as you describe.. it can also cause the engine to load up when idling thru a no wake zone gliderskiers carbs recommendations are much better but i would even go up another size from there.. your screw settings are best at 1 turn out or less.. the mikuni carbs are very easy to tune.. but first you must understand how each circuit operates and work together.. ive added a pdf file of the sbn tuning manual for you to read.. it makes it very straight forward on tuning..
maybe try a 130 pilot, 150 main with the screws 1 turn out and lean out from there.. and if the screws end up being a 1/4 turn out but it runs fantastic, leave it alone.. rich is good but sounds like youre too rich.. you could honestly probably leave the main jet and high speed setting alone and focus on low speed.. but your main jet and high speed may be affected as well..
you can try and go off other peoples carb adjustments and be close but the reality is every ski is different and most carb settings are not the same..
the other issue is the unrestricted stinger line which isnt super important.. without restricting the stinger water youll lose some off idle response but its not horrible.. a friend has a round nose with a mod b pipe and no flow control valve and it runs fine.. but this will definitely help..
