Re: stabil and poor starting
first, don't use ether on an outboard; it burns too hot and without oil and can damage the motor.
I got a used 150 Yam this year so you have a lot of the same things going on that I do or did.
If there is a fuel/water seperator (looks like an oil filter in the fuel line, outside the motor) change it; if not get one. Go for the "big blue" Yam filter.
I hear the swishing noise too; it's from the tank (under-deck tank in a Sea Hunt). It does not necessarily mean you have an air leak in the fuel line. If you do, it affects running more than starting.
Doubling the stabil is "shock treatment" and won't hurt anything. PS I'm a cook and I always double the spicey ingredients. And double it again.
I'd suspect the old gas in your new boat. Do you know about the history: did you buy a tank of old gas? how old, and how often did the PO run the boat? You might consider getting rid of all the old gas (if you haven't run it out already) and start new; if not, and the existing gas looks OK, either run it out or dilute it with fresh. gas >60 days old is falling apart (phase seperation) and you had it in the hot months of August and September.
If you are adding the filter, it's a good opportunity to pump some of that gas into a clear bottle and inspect it; ought to do it anyway before your next restart attempt. No telling what you're dragging through the system.
If there's an old filter, dump it in a clear bottle, too, let it settle and inspect. Do not reuse filters.
Keep the tank topped off.
Consider the possibility of water getting into the tank especially if you have a through-hull vent about the size of a quarter. Bypass that system.
After that, you're into the carbs and stuff under the hood. Tons of advice and info on these boards about that; you can decide what you want to tackle or what you want to send out to a Yamaha mechanic. If you (or the PO) didn't have the plugs replaced and carbs done, you probably should now before it gets worse. And change your water pump while you're at it.
Good luck with the transition from windbag to stinkpot. you have to shift your focus from what's in front and above you to what's behind you; that's where it all happens.
